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


DHutch

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Could I suggest that now you have received so much useful advice without coming to a firm conclusion that you consult a reputable paint manufacturer directly for their opinion?

 

I have over the years had brilliant help and advice from Leigh's Paints in Bolton, now taken over by the global company Sherwin Williams. They made the paint for the Forth Rail bridge refurbishment and for JCB I believe.

I have used their epoxy paints for many years and had no failures at , all, often on old rusty hulls.

 

You could also look at the process that Debdale Wharf use, their zinc coating treatment carries a 10 year guarantee on the outside of hulls!

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9 hours ago, DHutch said:

 

 

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?

No. There is nothing that will chemically eat its way through 3-4mm of black rust in a sensible time which won't also eat its way through the steel hull at nearly the same rate. If you air chisel or needle gun the black rust back to the point where the surface is mostly clean apart from down the pits then zinc phosphate/phosphoric acid will eat through the remaining rust and stop those sites being the starting point for rust under your new paint. Unlike Vactan which won't stick well to the clean bits.

 

If you can't get the black rust off, you are going to have to encapsulate it - no point trying to treat it as you won't go through it. You need something which sticks well to the rust and also forms a very good barrier so that no water and air can get through. If they do, the rust converts to red rust, swelling in the process and getting weaker. This forms a line of weakness behind your paint and the whole lot falls off as a sheet eventually.

 

Vactan and Fertan work best when the rust is exactly the right thickness. Too thin (or areas which are too clean) and they don't have any rust to convert and form a bond to. Too thick and there is a layer behind which hasn't converted, too thick and you will have a layer that doesn't convert. You can get away with the latter to a point but have to be aware that when the coating gets breached, rust will run away behind it.

 

Another option would be to paint the bad areas with rust encapsulating primer first. There are different types. Single pack red oxide types are easy to apply and fairly cheap. I have never had much luck getting the high build properties they claim as the second coat always tends to dissolve the first., so where I have used them, I have then overcoated with another primer with a different solvent. The alternative would be an encapsulating epoxy primer such as POR-15 which is expensive but I can vouch for its effectiveness. I wouldn't want to do a whole boat with it (way too expensive) but it would do the areas which can't be prepared back to clean metal, and then paint over it with whatever you are generally using.

 

Alec

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2 minutes ago, Tracy D'arth said:

Could I suggest that now you have received so much useful advice without coming to a firm conclusion that you consult a reputable paint manufacturer directly for their opinion?

 

I have over the years had brilliant help and advice from Leigh's Paints in Bolton, now taken over by the global company Sherwin Williams. They made the paint for the Forth Rail bridge refurbishment and for JCB I believe.

I have used their epoxy paints for many years and had no failures at , all, often on old rusty hulls.

 

You could also look at the process that Debdale Wharf use, their zinc coating treatment carries a 10 year guarantee on the outside of hulls!

Leigh Paints are very good, as are HMG Coatings (who are still independent). If you do speak to HMG, I suggest ringing HMG South in Andover as they are far more switched on than the head office and I have always had a much better answer.

 

The process used by Debdale Wharf is metal spraying. It is an extremely effective process, used in offshore applications including wind turbine towers in the splash zone and by the Chinese navy for the South China Sea fleet. However, to get the sprayed coating to bond, you absolutely have to grit blast the surface first and it has to be completely clean too. When I am having parts sprayed (mainly bits of old Land Rover at the moment) I blast them first, apply phosphoric acid, then re-blast immediately before they are sprayed (literally out of the blast cabinet and into the spray booth). I do the blasting for the person doing the spraying. Metal sprayed coatings provide galvanic protection, like a sacrificial anode. However they do not fully seal so you need a sealing coat over the top. Again, not a problem but it is very depressing if you don't realise this and leave parts lying around for a few months only to find a rust film on the top like I did the first time. Obviously this can all be done on a boat hull as Debdale do it, but it is not a DIY job.

 

Alec

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15 hours ago, DHutch said:

It would be faster to cut the whole section out in drydock and weld a new 5ft of baseplate in!

 

14 hours ago, DHutch said:

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?

 

If you have 3-4 mm pits in the top surface of a 30 year old baseplate, and quite possibly pitting from below as well, I have to ask, how thick was the baseplate originally? The option of complete replacement, or maybe overplating might be one to consider. (Although ovetplating would still leave you with a rust problem on the existing plate).

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2 hours ago, David Mack said:

 

 

If you have 3-4 mm pits in the top surface of a 30 year old baseplate, and quite possibly pitting from below as well, I have to ask, how thick was the baseplate originally? The option of complete replacement, or maybe overplating might be one to consider. (Although ovetplating would still leave you with a rust problem on the existing plate).

Quite so.

That is our issue. Boat was blasted, then on inspection we were advised to have over plate. We were 3 weeks from having to go back to Aus for work and 5 days post a bereavement. Brain not in thinking mode. We did the overplate and it was well done apparently on subsequent survey.

But the old problem of rust from the top down on a boat that can let in water at times remains. We keep it as dry as we can but the hold still sometimes gets wet.

 

we should have cut out and rebottomed but with back cabin engine room and extension to refit it would have been awful, and at the time it was a 4/12 home.

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9 hours ago, David Mack said:

If you have 3-4 mm pits in the top surface of a 30 year old baseplate, and quite possibly pitting from below as well, I have to ask, how thick was the baseplate originally? 

It's a 10mm base plate, effectively zero pitting on the outside, so we're not worried yet. But stopping the pits getting deeper is very much on the agenda. 

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Have you tried playing a MAPP torch over the rust and then brushing it off with a normal hand wire brush? I have found that quite effective as the difference in thermal expansion between the steel and the rust causes it to ping off. It can take a few goes, and obviously no good around anything flammable, but I used it for de-rusting some heavily scaled 6mm thick steel parts to good effect, and of course it doesn't need electrical power, and it does leave the surface dry so you know you are not painting over any residual wet rust.


Alec

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13 hours ago, Tracy D'arth said:

Could I suggest that now you have received so much useful advice without coming to a firm conclusion that you consult a reputable paint manufacturer directly for their opinion?

 

I have over the years had brilliant help and advice from Leigh's Paints in Bolton, now taken over by the global company Sherwin Williams. 

We use Lieghs paints (SW now) and have done since the boat is new, in part because my grandfather is a Boltonian and used them decades before hand. 

 

We will certainly speak to them, however it's also good to get ideals and thought from here. Especially as what works commercially isn't always compatible with or the best option for the DIY boat application. 

 

It does however sound like a really searching thin paint, and encapsulation is the best route, rather than mechanical or chemical removal or conversion. 

 

 

Daniel

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