Corrosion underneath paint in newly painted bilge

[MardyBum]

Evening all. We're currently in the process of gutting and redoing out boat. After clearing out the gunk and rust scale from our bilge bays we applied two coats of vactan, let it dry for several days, then two coats of bilge paint. All seemed well.

 

Now, three months later, the paint is bubbling and it looks like some micro-pitting and corrosion, or at least rust to some level is evident beneath the paint, fairly pervasively throughout the bays, as per the example in the pictures (in order - painted; scraped back; wiped clean).

 

Is there anything I can realistically do, short of shot blasting or grinding everything back to bare steel, to completely eliminate this?

 

Thanks folks.

Edited October 29, 2023 by MardyBum

[David Mack]

Sorry to tell you, but this looks like the result of insufficient preparation before painting. You really do have to remove as much rust as you can before applying rust converters like vactan. Even so these are simply a second best solution to completely cleaning the steel.

Grit blasting is best, but completely impractical for a DIYer. Next best would be needle gunning. You can hire a needle gun and compressor, or maybe an electric one (probably 110V so you need to hire a transformer as well), but the work is noisy and dirty so not necessarily something you will be allowed to do on your mooring. Otherwise it's down to grinding with a flap wheel in an angle grinder, which will clean off the metal surface, together with use of a wire brush on an angle grinder to try and clean out the pits. Maybe also a hammer and pointed chisel in the pits. After that use a rust converter. I would also think a coat of decent primer under the bilge paint, rather than just relying on the vactan.

[blackrose]

I would stick to the wire wheel or cup on the angle grinder rather than flapwheel disc. All you're trying to do is get rid of the paint so you don't want to use anything that will also remove good steel. 

 

I also used Vactan in my bilges and a year later the rust was back so personally I'd just avoid it. I think it's rubbish but others seem to love it. My prep was very good. I used to do this horrible job in a boatyard for 2 years so I do know a bit about good steel prep. 

 

Next time I'll do the job properly and use a few coats of Jotamastic 90 direct to the steel. Suggest you do the same.

[PeterF]

If this is below the waterline I am not sure that the Vactan will completely cure / dry out due to the steel being cold from the low water temperature  even if the air is warm. It may fully cure / dry out but it will take an extended time to do so and may well get overpainted before it has had time to cure. I think that relying on the Vactan here as a primer is not optimal.

[blackrose]

5 hours ago, PeterF said:

If this is below the waterline I am not sure that the Vactan will completely cure / dry out due to the steel being cold from the low water temperature  even if the air is warm. It may fully cure / dry out but it will take an extended time to do so and may well get overpainted before it has had time to cure. I think that relying on the Vactan here as a primer is not optimal.

 

Maybe that's it, but others seem to have had good results using Vactan on engine room floors. The Vactan I used had fully cured as far as I could tell and it was used within its 18 month shelf life. Given the relatively short shelf life of Vactan I think there may be some product variability between batches.

 

There's nothing wrong with the tannic acid used as a rust converter in Vactan, what I have a problem with is the water based polymer matrix that it's contained within. I agree with you that it's really not an optimal steel primer, especially for areas that are prone to condensation and puddles of standing water.

[john.k]

Rust is a water bound compound ...its easily removed by flame spalling ,then wire brushing the powdery residue.......there is no need to heat the parent metal red hot ,the rust is a poor conductor of heat and becomes white hot instantly under the flame ...........I have many years experience of blasting and painting steel .........if you MUST paint over rust ,then the first coat needs to be a rust converter ...and these dont work over more than very light surface  rust................soon as the little pits form ,then rust will come up under paint ,even if the steel is surface ground shiny......the  pits are still there ,and spiders of rust will spread under the paint within a year.

[MardyBum]

Thanks for the advice.

 

@David Mack I am sure you're right regarding the prep in these areas. I was very conscious of giving the vactan enough time to try, and the areas of good steel with minimal corrosion/pits seems fine - vactan and paint both adhered well.

 

@blackrose Funnily enough I was looking at Jotamastic 90 the other day, but assumed that the surface would need prep beyond what is practical for us (i.e. grit blasting).

 

@john.k What is flame spalling, as a technique? Google was not helpful!

[blackrose]

Yes, so it's much better to remove the rust properly and you can get it out of pits with wire wheels & cups on an angle grinder and out of corners with wire brush fittings on a drill.

 

A good surface tolerant epoxy such as Jotamastic 90 will cope with flash rush or gingering where bare steel has been water blasted and rust to a surface prep of ST 2.5 which looks like a brown tinge. I painted the bottoms of my badly pitted stern deck lockers which have permanent standing water in them about 7 years ago. When I finished my prep there was still a slight tinge of brown rust colour in the pits, but after 4 coats of Jotamastic the rust has still never returned.

Edited October 30, 2023 by blackrose

[john.k]

Pretty sure the BOC website will have what you need on flame spalling .....or it maybe called flame derusting.

[jonathanA]

hmm introducing an oxy-acetylene flame into an engine hole... what could possibly go wrong...   sorry go BOOM! 

 

you'd be a braver man than me (although thats not saying a lot) 

[BEngo]

Flame cleaning for rust is basically playing a just reducing oxy-acetylene flame across the rust.  It is effective, but not suited to typical narrow boat engine space, unless you are on really good terms with the local fire brigade.

 

Now is not really  the season for painting bilges  Too much condensation about.  OK if you can find a damp surface tolerant primer.  Then you need a couple of coats of undercoat and top coats of something like Danboline.

 I suggest giving it another clean up as suggested above then applying some Fertan in accordance with the instructions and repeating the treatment every month or 6 weeks until the water has warmed up next summer.  That will help get the Fertan into the base of the pitting as well as holding the rust at bay.

 

N

 

[blackrose]

Given what a chore getting all that paint and rust off manually is even with the aid of power tools (and it really is hard work), I repeat what I said earlier. The only way to do it properly and prevent the rust coming back for many years is to paint it with a good, surface tolerant epoxy. Forget about any single part paints for wet or damp bilges.

 

Anyway it's up to the OP. It's their boat...

Edited October 30, 2023 by blackrose

[MardyBum]

Update: Using an angle grinder with a wire brush cup and going with the Jot 90. They do a winter grade version so not too worried about application temps, for the moment at least. I can't do anything about the bays I've already done (partially built over them) but going forward it's epoxy all the way 👍🏻

[David Schweizer]

I am not a fan of Vactan, never ever having had much luck with it. When I painted my engine bilges, I cleaned as much old paint and rust off, and after de - greasing gave it two good thick coats of what was called Finnegans Smoothrite in those days. Finnegans had assured me that providing I let it cure completely, the paint was both heat and oil resistant, after ten years there was no sign of deterioration. Smoothrite is still made, but under a new name of Hammerite Smooth.

[MardyBum]

Update 2: Onto 4th and 5th bays now with the Jotamastic 90 (winter grade). Seems to be taking and curing well, I know the temperature and humidty outside at the moment is not ideal, but I've got around this with a couple of oil rads a dehumidifier. So far so good, although I guess time will tell.

 

Taking a lot of time with prep as I really don't want to have to do this again anytime soon! Angle grinder and combination of wire wheel and stripping discs seems to work well. Have had to use a hammer in places to loosen thick scale, then going over any polished metal with the sander to key up the surface. Lastly a good vacuum (or several) and a degrease/wipe down with pure isopropyl alcohol before painting.

[dmr]

Thats a lot of work so hope this is a good and valuable boat 😀.

Dunno how old you are but this will likely outlast you, and also outlast a lot of newer boats.

Nice job, well done.

[beerbeerbeerbeerbeer]

Just need to paint the other side now,

any plans to black/paint the outside of the base plate?

 

 

[MardyBum]

9 hours ago, beerbeerbeerbeerbeer said:

Just need to paint the other side now,

any plans to black/paint the outside of the base plate?

 

 

Did that early summer last year after we bought the boat as it was long overdue - went for the SML surface tolerant 2 pack epoxy, 3 coats. It was more of a stop gap though as there's quite a bit of welding that needs doing and there were some signs of MIC (only realised this after the fact, however).

 

It'll be coming out this summer for grit blasting, welding, additional anodes and then proper 2 pack. Might go for the zinc coating, haven't decided yet.

 

@dmr Thanks! Hoping to do all this properly now, once, so I don't have to worry about it!

[Eeyore]

9 hours ago, MardyBum said:

Did that early summer last year after we bought the boat as it was long overdue - went for the SML surface tolerant 2 pack epoxy, 3 coats. It was more of a stop gap though as there's quite a bit of welding that needs doing and there were some signs of MIC (only realised this after the fact, however).

 

It'll be coming out this summer for grit blasting, welding, additional anodes and then proper 2 pack. Might go for the zinc coating, haven't decided yet.

 

@dmr Thanks! Hoping to do all this properly now, once, so I don't have to worry about it!

Don't fit more than the recommended number of sacrificial anodes; "over protection" can cause coating to lift.

[ditchcrawler]

13 minutes ago, Eeyore said:

Don't fit more than the recommended number of sacrificial anodes; "over protection" can cause coating to lift.

But who's recommendation do you listen to. My boat came with 4, 3 at the bows and two at the stern, When blacked at Braunston I was persuaded to have another on each side, Last time it was being blacked the welded working on the boat beside us said I needed 5 on each side in total.

[Alan de Enfield]

4 minutes ago, ditchcrawler said:

But who's recommendation do you listen to. My boat came with 4, 3 at the bows and two at the stern, When blacked at Braunston I was persuaded to have another on each side, Last time it was being blacked the welded working on the boat beside us said I needed 5 on each side in total.

 

Did the folks advising you to 'have more' happen to sell anodes and offer a fitting service ?

 

 

 

 

 

Summary : 

Don’t add additional anodes to the system – an anode system needs to be planned correctly, and if you have too many anodes it will overprotect, and instead of protecting your components, they will wear away.

 

 

Can You “Over Zinc” a Boat?

Over zincing happens when there is too much anodic protection. Having an excessive number of sacrificial anodes installed on your boat can lead to unintended consequences. Boats with fiberglass hulls are less likely to be affected by over zincing, but wooden and metal hull boats are particularly susceptible to the negative effects of over zincing which include:

• Burnback and discoloration
• Paint delamination and flaking
• Caustic wood rot

So, how much sacrificial zinc does your boat need? Cathode protection systems vary depending on the type and size of your boat and where it will operate. Typically, sacrificial anodes should have 1 to 2 percent of the surface area of the metal surface it needs to protect.

[Sea Dog]

12 minutes ago, Alan de Enfield said:

Typically, sacrificial anodes should have 1 to 2 percent of the surface area of the metal surface it needs to protect.

Trouble is, how much is that on a freshly blacked narrowboat? There really shouldn't be any exposed metal on the hull side, and I understand that an anode only protects in line of sight so that rules out the baseplate whether blacked or not. Ergo, other than a small one to protect the prop and shaft, one could argue (as some do) that a freshly blacked boat doesn't need anodes. By the time it comes around again it might, but is this partially because it was over protected from the off? 

[MardyBum]

This is all very interesting, I was not aware you could 'over-anode' - learning every day! I will do a bit more research.

 

For info, we're 48ft and currently have - on each side - two at the bow, one more worn than the other (I think they just left the oldest one on), one midship and two at the stern (again, one significantly more worn than the other).

 

My plan was to remove the anodes midship and the really old at bow/stern, and replace with two either side of the midship so the spacing between anodes was a bit more even, roughly 12ft apart.

 

I welcome discussion/suggestions/scepticism/outrage... 😂

 

 

12 hours ago, Sea Dog said:

Trouble is, how much is that on a freshly blacked narrowboat? There really shouldn't be any exposed metal on the hull side, and I understand that an anode only protects in line of sight so that rules out the baseplate whether blacked or not. Ergo, other than a small one to protect the prop and shaft, one could argue (as some do) that a freshly blacked boat doesn't need anodes. By the time it comes around again it might, but is this partially because it was over protected from the off? 

 

Based only on my observations of our base plate after it had been pressure washed, I would question this. The base plate had some pitting and corrosion all along the length, with the exception of the areas immediately adjacent to and a few of feet either side of the existing anodes - the steel seemed almost unbelievably smooth and flawless.

 

I came to the conclusion that this was evidence of the anodes doing their job and protecting our base plate too 🙂

Edited January 16 by MardyBum

[ditchcrawler]

3 hours ago, MardyBum said:

Based only on my observations of our base plate after it had been pressure washed, I would question this. The base plate had some pitting and corrosion all along the length, with the exception of the areas immediately adjacent to and a few of feet either side of the existing anodes - the steel seemed almost unbelievably smooth and flawless.

 

I came to the conclusion that this was evidence of the anodes doing their job and protecting our base plate too 🙂

I came to the same opinion when I looked under mine in Middlewich dry dock as it hadn't been painted underneath the time before as you can't get under it there. I did post a picture at the time.

The anodes are down above the bostock right at the bottom of the photo.

Edited January 16 by ditchcrawler
I like adding bits

[MardyBum]

Evening all - still slogging away at the bays down the boat.

 

Came across a deepish pit on the hull side (6mm steel), below the water line (around 2.5mm max deep I reckon, inch or so wide) and wanted a steer/sanity check on options.

 

As this is inside the boat, I was considering cleaning the pit out to shiny steel (done) and then just filling with an appropriate 2-pack filler and painting over with the epoxy.

 

Other option is to get it welded, but for an internal pit of this depth, is it necessary - f I can stop the corrosion and fully seal it?

 

Thanks folks.

 

 

 

Edited March 17 by MardyBum