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Treating black rust on the inside of a heavily pitted bilge.


DHutch

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2 minutes ago, dmr said:

 

Sounds like you know about this stuff, so a few comments/questions?

 

1 If its inside the boat I don't think the epoxy needs a single pack topcoat, and leaving the epoxy uncoated makes it easier to repair/recoat in the future.

 

2 Is there a "ready made" phosporic acid-zinc phosphate "paint" available?

 

3 Are you talking about the likes of Zinga/Galvafroid here? Lots of people rave about ZingA stuff but I had not heard that it chips easily. I need to do my baseplate soon, should I go for epoxy or Zinga + epoxy?

 

I do know a fair bit about it. I used to run the coatings group for a research institute. I am not to the level that some of my technical experts are/were, but I did pick their brains a fair bit on the subject of good anti-corrosion systems that would work on steel. In answer to your specific questions:

 

1. Epoxy does not need a topcoat inside the boat. I was actually referring to a 2-pack polyurethane rather than single pack. This will last decades, even wet. However, it is nasty to apply, should not be handled without suitable PPE and is therefore best avoided unless you have access to this. Leaving it uncoated is, as you state, much easier to then repair/recoat.

 

2. It's more of a watery fluid than a paint. Yes, there are several brands. When I bought mine I went for Eastwood's After Blast as it was the only one I could find but I have since found much cheaper versions, which I would name if I could remember where I saved the information! It is expensive but a little goes an extremely long way and it does not go off.

 

3. Yes, I am talking about Zinga, Galvafroid and various aluminium primers. They stay quite soft and brittle. I have found that you can wire brush them off with a brass brush and that seems to deal with some of the issue, but they are nowhere near as mechanically strong as a cured epoxy or polyurethane, or even an alkyd. The aluminium ones also have a tendency to form a slightly waxy surface which needs a scotchbrite over it to get the next coat to bond.

 

Alec

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2 minutes ago, TheBiscuits said:

Fertan, but you need to rinse off the residue and dry it out again before painting.

 

A wet & dry vacuum helps if you have enough access.

 

Right. Which is presumably the case with all of them? Either they contain a primer, or they need rinsing?

 

Obviously as you suggest, while rinsing and drying is easy with a part in a workshop on a nice day happy days, hose and airline it off,. lovely. When it's part of the boat however, you don't want to rinse it into the canal or onto another part your trying to paint, or the bilge, it's more faff. If maybe in this instance, less faff than starting again 2years later!

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2 minutes ago, DHutch said:

However, we understand the importance of a good mix, and mixed all our two part paints (epoxy and the acrylic urethane topcoat) carefully and accurately using a microbalance and the manufacturers guidelines, including running out of the two parts at about the same time which is always a good indication!

 

Did you thoroughly stir the base before measuring quantities?  I'd use a mixing paddle on a drill for five or ten minutes before measuring the amount I wanted of the base part.  The curing agent shouldn't need mixing before measuring and certainly not with the same paddle ...

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2 minutes ago, TheBiscuits said:

 

Did you thoroughly stir the base before measuring quantities?  I'd use a mixing paddle on a drill for five or ten minutes before measuring the amount I wanted of the base part.  The curing agent shouldn't need mixing before measuring and certainly not with the same paddle ...

 

:clapping:

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18 minutes ago, DHutch said:

While prep'ed about as well as we could, given the initial conditions, there was very little bright steel. And the best and least pitted areas where maybe only 50% bright steel, and are the area the paint had stuck best! So while under the concert, it wasn't what was happening here. 

 

As per the 'Misuse of rust converters' thread I have yet to find this mystic 'easy to remove rust' as most rust is in the hard to reach spaces of the world (for obvious reasons) and even if access is good, grinding anything back to bright takes an age!

 

https://www.canalworld.net/forums/index.php?/topic/104614-the-misuse-of-rust-converters-the-vactan-myth/page/2/

 

So having ruled out 1.  (Which also includes a metal loaded epoxy) we're onto option 2.

 

 

I'm warming to the idea of phosphoric acid and zinc phosphate as an option. What are we looking at here, in terms of a process, and required materials. Aka, where do we get the stuff and what to do we with it?

 

 

Did it go black when you applied the Vactan? If not, it may be cleaner than you think. Steel is only bright when it is both oxide-free and smooth; after cleaning it may well be clean enough not to have enough rust on to convert but not actually shiny. However, if it went black then scrub that thought. If someone finds the easy to remove rust, please could they let me know too! Vactan works best if you do nothing more than a hand wire brush and a wipe down to degrease - if the rust came off with that I would be extremely happy!

 

Phosphoric acid and zinc phosphate works really well on the kind of surface prep it sounds like you have been doing. Process is easy - clean off as you have been doing, paint on the watery fluid with a paintbrush, mop off any excess after about 10mins and allow to dry thoroughly. Then prime/paint as normal. If I remember the name of the cheaper version then I will add it.

 

Alec

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Just now, TheBiscuits said:

Did you thoroughly stir the base before measuring quantities?  I'd use a mixing paddle on a drill for five or ten minutes before measuring the amount I wanted of the base part.  The curing agent shouldn't need mixing before measuring and certainly not with the same paddle ...

We mix the base well before measuring, if typically carefully and vigorously with a stick rather than drill, due to the cleanup issues with two pack. 

 

The ali epoxy base is so thick you can stand a spoon in it and it be there days later. When fresh from the factory a few days earlier, we just use it. After about a year it with settle with a 5mm layer on top, and again we beat that into it well. 

 

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2 hours ago, roland elsdon said:

Have had exactly the same in our hold. At the back where it can get water lying. 3 years ago i ground off, with wire wheel in grinder derusted, double primed, top coated in gloss and then waxoiled.


lifted shuts this week and failures everywhere. chipped it off, fertanned it, and waxoiled on top. Cant be any worse.

 

the next bay i had treated 2 years ago but instead of paint and primer i just waxoiled. This looks intact.

 

so maybe the answer.

 

Admittedly im dealing with a 20 year old bottom ...

Infuriating isn't it! This bottom is 30 years old. 

5 hours ago, Neil2 said:

I like Owatrol.  I know some are sceptical but I've used it in situations like this where it's just not possible to do anything more than remove the loose rust and it works for me.  Ever since the demise of Trustan I don't think there's ever been an effective rust converter.

 

And maybe this is the answer. Forget about trying to get any paint to stick, and go down the Owatrol Oil, Waxoyl, Dinatrol route?

 

Early post about Waxoyl being porus noted, if slightly supprised by that comment?

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2 hours ago, dmr said:

If you can get almost to bare metal (and a needle gun should do that) then apply a surface tolerant epoxy primer direct to steel, after suitable cleaning.

I wish the needle gun got it back to bright. 

 

Often it's very good, if the rust is in the early stages, nice loose fluffy red rust, off like a flash and a lot better at taking the failed coating off to access it that a wire wheel. 

 

However the black rust when it's had limited oxygen, deep at the bottom of these pits, is good and hard the surface so 3D at 3mm+ deep the gun seems to struggle.

 

This also not helped by the size of the task, as the compressor can't keep up with gun, so you end up running at lower pressures and powers even if you wait for it to get back up for the harder bits and or a final blast. 

 

It's 'only' machine mart Clarke Air model, and I'm sure an industrial model with a 5+hp compressor to match and sharp needles would do more. But it certainly can't get this area of the boat upto good enough to put even surface tolerant epoxy ali onto. 

 

Daniel

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6 minutes ago, DHutch said:

I wish the needle gun got it back to bright. 

 

Often it's very good, if the rust is in the early stages, nice loose fluffy red rust, off like a flash and a lot better at taking the failed coating off to access it that a wire wheel. 

 

However the black rust when it's had limited oxygen, deep at the bottom of these pits, is good and hard the surface so 3D at 3mm+ deep the gun seems to struggle.

 

This also not helped by the size of the task, as the compressor can't keep up with gun, so you end up running at lower pressures and powers even if you wait for it to get back up for the harder bits and or a final blast. 

 

It's 'only' machine mart Clarke Air model, and I'm sure an industrial model with a 5+hp compressor to match and sharp needles would do more. But it certainly can't get this area of the boat upto good enough to put even surface tolerant epoxy ali onto. 

 

Daniel

 

I have had good results with a needle gun and thought (hoped) tthat hey were a close second best to blasting, but I have not done any serious deep rust. The black rust is interesting and I would like to understand it more. I had a lot on the side locker floors and it was Very hard, the usual wire cup brush sort of polished it but didn't really shift it. It was almost in thin sheets and a hammer and cold chisel did a better job. 

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As I understand it, black ferroso ferric rust (Fe3 O4) is formed rather that conventional red rust (Fe2 O3) when there is a shortage of oxygen, such as under a robust but failed paint surface, or if you have water trapped under oil or the like, and rather than coming of nicely it's a right pig! And it's magnetic...

 

I'm not a chemist, but I can vouch for it being a pig to remove!

 

Maybe we should sharpen our needle gun, and or get a 3-4hp compressor!

 

 

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Also just and overall comment to say a huge thanks to everyone who has commented on the thread so far.

 

I've tried to keep up with the replies, and answer the questions raised, but if I've missed the odd one let me know!

 

 

Daniel

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12 minutes ago, DHutch said:

As I understand it, black ferroso ferric rust (Fe3 O4) is formed rather that conventional red rust (Fe2 O3) when there is a shortage of oxygen, such as under a robust but failed paint surface, or if you have water trapped under oil or the like, and rather than coming of nicely it's a right pig! And it's magnetic...

 

I'm not a chemist, but I can vouch for it being a pig to remove!

 

Maybe we should sharpen our needle gun, and or get a 3-4hp compressor!

 

 

I was once a chemist and agree with your assessment. Fe3O4 is magnetite, Fe2O3 is haematite.

 

Magnetite is deliberately formed as the protective blueing on tools, guns etc. because it is hard and won't chip off. This is not helpful in this situation! Eventually if exposed to air it will turn to haematite and become easier to remove but I suspect you don't want to wait that long...

 

Coincidentally, the original millscale is also mainly haematite as it forms preferentially at high temperatures, but it also includes quite a lot of burnt in carbon from the lubricant on the rollers. Millscale is similarly hard to remove, until water gets under it, converting the lot to haematite and falling off in big sheets, bringing your paintwork with it.

 

Alec

Edited by agg221
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6 minutes ago, DHutch said:

As I understand it, black ferroso ferric rust (Fe3 O4) is formed rather that conventional red rust (Fe2 O3) when there is a shortage of oxygen, such as under a robust but failed paint surface, or if you have water trapped under oil or the like, and rather than coming of nicely it's a right pig! And it's magnetic...

 

I'm not a chemist, but I can vouch for it being a pig to remove!

 

Maybe we should sharpen our needle gun, and or get a 3-4hp compressor!

 

 

 

That's interesting and worrying. So if epoxy detaches but remains otherwise intact then that has potential for making hard black rust?. I think I have seen this but will now be watching carefully.

 

I think needle guns like a big air supply.

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4 minutes ago, dmr said:

 

That's interesting and worrying. So if epoxy detaches but remains otherwise intact then that has potential for making hard black rust?. I think I have seen this but will now be watching carefully.

 

I think needle guns like a big air supply.

Black hard rust itself is not a bad thing. If you can dry it out temporarily and soak it in oil or wax then it is fairly protective - see deliberate use on guns and tools. It's when it converts to red rust and the epoxy all falls off you have real problems. The final paragraph in the scope of work linked here:

 

https://gtr.ukri.org/projects?ref=34774

 

may be of interest.


Alec

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It wouldn't be impossible to get it blasted. You'd need to put up some plastic sheeting around the rest of the area, wrap up that propshaft, remove anything that can be got out of the way, and shovel out & hoover up all the grit when finished.

 

If you can get good epoxy onto freshly blasted steel and follow it up with a couple more coats you will never have to worry about it again, it will outlive you. The epoxy works its way into all the microscopic nooks and crannies of the rough surface. The result is an astoundingly strong bond, you're basically making a thin layer of composite. You'll never get the paint off except by abrading it away.

 

Alternatively, do the same thing you did before but without the Vactan. By the time you've got to a surface that's been needled and gone over with a wire wheel, you shouldn't need a rust converter. Anything you have to put on and rinse off at that stage is probably just going to worsen adhesion of the epoxy.

 

The Clarke needlegun isn't bad, but for pitted areas it's often necessary to take the needle attachment off it and use the single-point chisel that comes with it instead. It's much better at knocking those last bits of black magnetite out of the bottoms of all the pits. The needles can often fail to get right in there, since too much of the available force is hitting the surrounding areas instead. For an area that gets wet, it's crucial that you get right down to sound metal in every pit, or there's bound to be water trapped between layers at the bottom of them.

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5 hours ago, Giant said:

It wouldn't be impossible to get it blasted. You'd need to put up some plastic sheeting around the rest of the area, wrap up that propshaft, remove anything that can be got out of the way, and shovel out & hoover up all the grit when finished.

 

If you can get good epoxy onto freshly blasted steel and follow it up with a couple more coats you will never have to worry about it again, it will outlive you. The epoxy works its way into all the microscopic nooks and crannies of the rough surface. The result is an astoundingly strong bond, you're basically making a thin layer of composite. You'll never get the paint off except by abrading it away.

 

Alternatively, do the same thing you did before but without the Vactan. By the time you've got to a surface that's been needled and gone over with a wire wheel, you shouldn't need a rust converter. Anything you have to put on and rinse off at that stage is probably just going to worsen adhesion of the epoxy.

 

The Clarke needlegun isn't bad, but for pitted areas it's often necessary to take the needle attachment off it and use the single-point chisel that comes with it instead. It's much better at knocking those last bits of black magnetite out of the bottoms of all the pits. The needles can often fail to get right in there, since too much of the available force is hitting the surrounding areas instead. For an area that gets wet, it's crucial that you get right down to sound metal in every pit, or there's bound to be water trapped between layers at the bottom of them.

I would agree with this. Having done quite a lot of blast cleaning with some fairly industrial kit, a couple of other observations to add.

 

Big grit is better at knocking off big bits but small grit goes down finer holes.

Even with the best blasting you can't always get everything out of every pit. You sometimes get occluded pits where there is metal above but rust has gone down the grain boundary within the metal.

My preferred approach when I get the option is as follows:

Blast with coarse grit to knock the lumps off

Blast with fine grit to get back to a clean surface - may still see a few dark spots down the pits but nothing red or loose at all

Immediately apply phosphoric acid to convert any remaining rust down the pits (note, phosphoric acid does not need to be washed off, just brush on, leave 10mins, wipe up any wet residue with a paper towel and then leave the damp surface to dry)

Leave overnight at least, and for up to 3 days

Re-blast with a fine grit, back to a clean surface with as few visible marks as possible

Coat immediately

 

I prefer to only do the final blasting on the area I can coat straight after - leave it more than an hour or two in dry, fine weather and it will flash rust on the surface. There are paint systems which will cope with that, but it's not as good as going on to the fresh surface.

 

The same approach would apply with a needle gun, wire brushes etc.

 

Alec

Edited by agg221
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15 hours ago, dmr said:

I think needle guns like a big air supply.

Yes. It claims 4cubic feet a min average, but that must be a low duty cycle! More like 10-15 while running I would guess. 

 

 

15 hours ago, dmr said:

 

That's interesting and worrying. So if epoxy detaches but remains otherwise intact then that has potential for making hard black rust?. I think I have seen this but will now be watching carefully.

 

We use epoxy ali/zinc everywhere and basically dont have such problems.

 

A ) If it sticks once, it's on for life. Ditto the thin 2pack blast primer which really does stick like mad 30+ years later.

B ) Usually coating failure is small, localised, and produces red rust, and is easy to clean off and overpaint. 

 

This is just a rare case where we have an area that has rusted usually badly, that we can't seem to get control of again.

 

Not only is it rusting under the epoxy, it's also damp often enough you get long term retained moisture under the epoxy. 

 

 

 

The main cause was that we stored lead ballast down there. Which mechanically damaged the coating, and created a pd that preferential corroded the steel. Like a sacrificial anode only backwards! Combined with crap access and the fact it's covered up most of the time, it got left too long and hence got this bad in the first place! However we no longer store lead there, haven't for over a decade, so I don't think that's a continuing problem. 

 

 

Daniel

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I've found the ordinary Owatrol oil to be excellent at stabilising rust, and have tried various treatments throughout the boat. It has performed nearly as well as armourguard st (a fairly bulletproof epoxy) but is even better on less well prepared areas. The only issue i had with it was on and around the stern deck when a brief shower left drop marks in the slow drying coating - they are near impossible to sand out!

 I bought some lead tetroxide and plan to mix it with the Owatrol next time to see how that goes.

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I tried owatrol 2 years ago on wire wheeled external cabin rust. Dried 3 days , 2 primer, 1 undercoat 2 top.

This summer i ground it off as there were rust bumps and tried fertan. 
Await results.

 

As for the bottoms im tempted to do what I did on Thaxted. Just pour the used engine oil under the shuts.

Trouble is the mess.

 

The wear from the shallow canals slowly meets the pits from above, but at least we can open up and clean out.

I suspect there are some real horrors under cabins where people fear to tread.

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On 23/09/2021 at 01:52, Giant said:

It wouldn't be impossible to get it blasted. You'd need to put up some plastic sheeting around the rest of the area, wrap up that propshaft, remove anything that can be got out of the way, and shovel out & hoover up all the grit when finished.

 

Yes, I mean we could blast it, but unless anyone can recommend somewhere with the kit that would do it for a reasonable cost in the NW, I still fear it's going to be prohibitive in terms of cost and mess. You have to sheet up the whole of the inside of the wheel house, from the floor to the folding sides, then the whole area below the floor which is about 3ft off the base plate, then stand in an 7ft deep hole shot blasting (your feet?) through the hatch in the floor. Which not damaging anything!

 

Not that it wouldn't be nice to blast it! 

 

And as suggested below, I'm sure if there off tiny bits, the epoxy would bridge over just fine.

 

On 23/09/2021 at 01:52, Giant said:

Alternatively, do the same thing you did before but without the Vactan. By the time you've got to a surface that's been needled and gone over with a wire wheel, you shouldn't need a rust converter. Anything you have to put on and rinse off at that stage is probably just going to worsen adhesion of the epoxy.

 

That was my thought first time right at the start of this, before we used Vactan much at all, and it actually exactly what we did do 15 years ago, but it failed.

 

Which is why we did it again, with Vactan, a few years ago, which has now failed again!

 

On 23/09/2021 at 01:52, Giant said:

The Clarke needlegun isn't bad, but for pitted areas it's often necessary to take the needle attachment off it and use the single-point chisel that comes with it instead. It's much better at knocking those last bits of black magnetite out of the bottoms of all the pits. The needles can often fail to get right in there, since too much of the available force is hitting the surrounding areas instead. For an area that gets wet, it's crucial that you get right down to sound metal in every pit, or there's bound to be water trapped between layers at the bottom of them.

Yeah, I mean it feels ok, and running at the designed circa 100psi  it's got grunt. However when the compressor doesn't have the whaft you end up at 40psi very quickly, and have to wait for it to build again, and it just takes all day. 

 

I was having a bit of a dig around after we did some more needle gunning, and while some pits where ok, maybe one in ten you could easy chip a 4mm thick 6mm dia chunk out, and maybe as many as 50% had a mm or so of the black rust left. 

 

I did actually bring the chisel but we ran out of time to try it, however with what must be 100s if not 1000s of pits you would never be able to do anything one an individual basis for the whole area, even 2sqm that's just days of work. It would be faster to cut the whole section out in drydock and weld a new 5ft of baseplate in!

 

Daniel

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Y

On 23/09/2021 at 15:49, BWM said:

I've found the ordinary Owatrol oil to be excellent at stabilising rust, and have tried various treatments throughout the boat. It has performed nearly as well as armourguard st (a fairly bulletproof epoxy) but is even better on less well prepared areas. The only issue i had with it was on and around the stern deck when a brief shower left drop marks in the slow drying coating - they are near impossible to sand out!

 I bought some lead tetroxide and plan to mix it with the Owatrol next time to see how that goes.

See that sounds really promising, and our surveyor also recommended it, and it's that pitted even rain marks don't worry me.

 

Roland's comments are less reassuring here. But have a read around the previous threads.

 

As I understand it, slightly contrary to it's name, it does dry to form a hard and coherent surface, that you can walk on, over paint, grind off again, etc. All bit of over a few days. I guess a bit like Danish oil drys. 

 

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On 22/09/2021 at 21:10, agg221 said:

Vactan/Fertan will work well so long as there is a layer of well bonded rust whereas phosphoric acid based treatments will work if the surface is mostly clean but with some rust down the pits on the surface. I prefer phosphoric acid with zinc phosphate added as it is very protective for a long time (I have left bare steel treated with this in a damp shed for 2yrs and it hasn't corroded). Then paint over this with a good single pack primer and topcoat.

 

 

On 22/09/2021 at 22:14, agg221 said:

When I bought mine I went for Eastwood's After Blast as it was the only one I could find but I have since found much cheaper versions, which I would name if I could remember where I saved the information! It is expensive but a little goes an extremely long way and it does not go off.

 

Alec

Ok. And although it is talk about being used after grit blasting, it will be able to treat/eat/prep its way into 3-4mm think black rust?

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