Keel preparation

ebbtide1

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I want to epoxy my cast keel and am happy with the required paint system. My question is: How is it best to prepare the surface. Blasting is not an option due to the location so I am stuck with grinding / sanding etc. Any thought swould be appreciated.
 

William_H

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Cleaning rusty keel

I have had some success on a friend's boat using an angle grinder with a rubber backing pad and sanding disc. I used a very course disc. Wear dust mask and safety glasses. What was left was treated with phosphoric acid (Rust converter) That was washed off and epoxy applied. The boat has not been lifted since so i don't know how well it all lasted. 10 months now.
It does seem to me that you will have rust reappearing no matter what you do. However this might minimise the rust or extend the life.
The problem here is that weed grows so fast that any rust just takes away a/f and seems to feed the weeds. good luck olewill
 

oldsaltoz

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I suspect the cause of most cast iron keels is the moisture trapped in them when coated.

Cast iron is porous and full of all sorts of contaminants and small voids.

when on the hard, the mass heats and cools slowly causing moisture to form on the surface and raise the moisture content.

Coating a moist steel surface, or a porous surface known to contain moisture, like cast iron, has it's obvious consequences.

Add to this the fact that cast iron will 'flash rust', in less than a minute you can see rust forming on the surface.

Conclution:
It Must be dry.
It Must be coated as soon a possible after cleaning.

PS. I have been seen applying protective coating to a keel with roller on the end of broom handle, just to make sure I was coated as soon a the sandblasters nozzle was 4 inches clear.

Good luck.:)
 

Little Rascal

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I'm just doing the same thing, lots of threads about it on here so it's worth a search.

The problem seems to be that with grinding the rust just gets shoved around and not removed like with blasting. A rust converter is a must I think.

Cast iron keels will have a high moisture content by definition so it's likely to be an ongoing battle - it just comes back!
 

Pasarell

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Keel Prep

Grinding is going to be way better than sanding although nothing like as good as blasting. As already said cast iron is a low grade metal with lots of impurities, voids etc. If you are only going to grind I would not bother with epoxy but go for a conventional primer like Primocon. Unless you remove all the rust, which is highly unlikely with a grinder, it will corrode again anyway and an epoxy wll be pushed off just as easily as a conventional primer. Difference is that Primocon has anticorrosive pigments in it whereas most epoxies do not. Other thing to do is make sure you remove all the salts from the surface. Good pressure wash before grinding and again after grinding. Allow to dry as much as possible with cast iron then a quick grind to remove flash rust before coating. In the UK environment flash rusting will be a much smaller problem than Oldsaltoz describes so you can completely grind the keel before starting to coat it. Better that than have all the small grinding debris getting in the wet paint. Film thickness is very important with underwater coatings so don't skimp on the number of coats. And avoid rust converters!!
 

vyv_cox

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The method used on offshore platforms when grit blasting is not available or appropriate is needle-gunning. Most hire companies keep the kit and they can do an excellent job. Their advantage on cast iron keels is that their action, like blasting, tends to open the pores in the metal whereas grinding tends to close them.
 
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