lxs602

Hi, My boat is being welded at the moment, bow and then stern. The welder is working on it on occasional evenings and weekends, so I estimate it may be done by the end of September. The paint on the sides and roof is mostly weatherproof (except around the windows), but is not in the best condition. While I have the time on hardstanding, I have thought about taking off the paint and doing a layer of weatherproof primer at least, if not the topcoat. If I pick days when it is not being welded, is this OK? Assuming the paint is given time to dry, before grinding or weldeding? Does this sound like a good idea (or a bad one)? I have heard about metal dust from boats being ground or welded landing on nice new (dry) painted boats and causing rust flecks. If metal dust lands on the prepared metal (or on the new (dry) primer after painting) would it cause a problem? Also, should you remove windows when painting the sides? Thanks for any replies.

Boatbuilder advised against vapour membrane suggesting it would trap water, which seems to be happening, and that the stove and sunshine on the metal roof would dry out the rockwool if not used. I don't know if others who have used vapour membrane may have just spent a lot of time to get a seal as required, as otherwise it seems to be counterproductive. Interestingly, a government paper from Sept 2021 titled Retrofit internal wall insulation: best practice says (emphasis added):

It has been difficult getting a perfect continuous barrier with the vapour membrane on the warm surface of the rockwool. Water has condensed on the metal roof despite the membrane, and seem to be trapped in by it, and then also not be dried out by the fire.

It was a serious comment... and I am inclined so. It is easy to waste time and resources this way, as well as irritate others.

I was surprised to have found a government guide called "Retrofit Internal Wall insulation: A guide to best practice" (Dpt of Business, Energy and Industrial Strategy) https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1019707/iwi-guidance.pdf On page 30, paragraph 45, "The addition of a vapour barrier inside Internal insulation (IWI) on solid walls used to be regarded as essential, partly due to inappropriate moisture risk assessment methods. However it is now agreed that in many cases, this may do more harm than good." It then mentions closed and open closed ventilation systems. I don't understand how this might apply to a canal boat. I have also seen new "intelligent" permeability vapour membrane advertised. Would this be a good idea? What with possibility of vapour being trapped between a vapour barrier and insulation?

That's worth knowing. Agreed. Perfectionism or fussiness should be avoided though.

It's been 10 years. How did it go?

Indeed so. I have just spent several days ripping out aged unhoused wiring running next to 1980s polystyrene sheets, so fire risk has been on my mind. Probably too late now. I'll have to go with a second coat of Hammerite direct-to-rust paint. Thanks again for the suggestions.

The vapour membranes available at stores nearby are polythene, flame resistance category E. Isn't that a fire risk? I was told by someone that you shouldn't put waxoyl over a layer of paint? I would have to use a second layer of Hammerite.

I will put rockwool on the ceiling, with a vapour membrane between the warm surface and wood ceiling panels.

Thanks for the suggestion. I will put a coat of waxoyl over the Hammerite. I'm sure it's not ideal, but this has all been very fast.

What should I use between the rockwool and the steel then? I am also using PIR enertherm boards (similar to celotex) on the walls. Wouldn't there be a galvanic reaction between the foil covering on the boards and the steel? As long as the steel is protected

I got it as dry as I could manage. Hope the stove being lit helped.

I'd thought that filling in the dots of rust still showing through the paint would reduce condensation on these spots and then rusting underneath and then flaking off the surrounding paint. I don't know if it would ioccur like this in practice. It is just my theory of course. Hence whether a second coat would give more comprehensive coverage. I thought tin foil as a moisture membrane between steel and rockwool was standard?

I removed the loose rust. It is designed for rusty surfaces but it's unfortunate if it's been reformulated. The VOCs are still rated high though.