Latest News: Forums Technical Repairing outer skin of hull (W World) Re: Re: Repairing outer skin of hull (W World)

#9635
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Well I have said this before, but please check other sources in case I have it around my neck…

Polyester resin works by cross-linking long-chain molecules which gives a chemical bond. The great thing about it is that the resin really does become a solid with molecular bonds that join around and through the reinforcing fibres, making a matrix that is extremely strong. That process only works while the underlying resin is fresh. This is why the gelcoat is only allowed to go partly off before the main resin lay-up begins on a new boat and for preference the whole of the lay-up has to be as continuous a process as possible (very large hulls excepted which I suppose are not worked on overnight, so a few hours between laminations must be acceptable).

The hull of No Disgrace (great name BTW) is “old” GRP, meaning that all of the molecular bonds are solid and “dead” and new polyester resin will not cross link with the old meaning that the patch you apply is never going to be part of the existing construction, but will lay on top of it. Given the exposure to water and UV, it will possibly/probably not be a permanent repair and at some point the gelcoat and resin of the outer repair will part company. Not sure if we are talking months or years, if it’s the latter then who cares?

Epoxy on the other hand is an adhesive (as in Araldite) and a mechanical bond is formed which, given a good abraded surface sticks extremely well. The downside is that polyester resin does not stick to epoxy, so the received wisdom is that Gelcoat will not adhere to it very well. I don’t have practical experience of this because I have never tried it, maybe others have.

My sources for all this BTW are two books by an American name of Don Casey, Hull and Deck Repair and Sailboat Refinishing. Good reading, well laid out and very empowering.

If painting /finishing is the worry, why not make it a feature, paint a big hole there with a shark’s head coming out, I’m sure I have seen that on a film…