Sticking stuff to a composite boat^

[Jim]

Not been involved with larger composite boats for a few years now (crikey - about 15) but I've never heard of osmosis resisting gelcoat. Gelcoat primarily makes the product look nice, it hides the laminate (and any mistakes made in the layup as a bonus) and helps provide a smooth finish where the shrinkage of the resin may otherwise create a textured finish. Of course that was before carbon and carbon/kevlar cloth were considered to look good, and the lower shrinkage of epoxy allowed manufacturers to do away with the gelcoat on many products.

Not sure why gelcoat doesn't air cure - is it to do with accelerators, or a by-product of the thixotrope? Adding wax which floats to the surface and excludes the air does the trick, if you buy gelcoat filler, it is pre-mixed with wax. In the first yard I worked in we hadn't heard of adding wax and used to use all weather tape over gelcoat repairs to make them cure (with varied results).

Flow coat really can be anything you want, thickened enough to be self levelling is preferred and for polyester I do believe we used to mix gel coat and laminating resin. Even just painting resin onto a cured laminate to get rid of unevenness is a form of flowcoating, with epoxy you could add silica or similar to get a nice consistency.

[al27]

There's something in the back of my mind re osmosis resistant gelcoats, but I'm not sure if its gelcoats specifically; about 20 years ago companies offering 10 year guarantee's against osmosis was suddenly the norm?? I think the chemistry is improving all the time, a lot of yachts are now built with an epoxy "gelcoat" below the waterline (epoxy is waterproof, polyester isn't) with a tie coat to the polyester backed laminate; this is a gelcoat in the sense that it's a layer that can be keyed for painting without cutting into the laminate. Epoxy gelcoats do exist; they don't have the quite shine or hardness of polyester gelcoats, and are mostly used in the above application or for moulds. There are also epoxies that are compatable with polyester gelcoats, or as I use, a vinylester gelcoat that's epoxy compatable. You don't get quite as good a bond as between poly/poly, but its the best compromise at the moment.

Al.

[Jim]

We used to do osmosis treatment for customers, if it wasn't too bad we would clean the antifouling and sand the hull below the waterline to reveal any blisters, and then use a specially ground stanley knife to open out any larger blisters we found. After leaving to dry out for a few weeks (under cover, sometimes stashed at the back of a friendly farm) we would paint the bottom with epoxy paint. Epoxy resin doesn't need a smelly colvent, epoxy paint does. One of my worst workdays ever was after a night out , in the middle of a hot summer, wrapped up in a boiler suit, on my back under a 23' boat on stacks of 4 concrete blocks (so pretty close to my face) trying to get a decent finish (the paint was nightmare) with a hangover and the paint fumes hanging thick around me. A couple of times I did have to wriggle out at high speed to go and throw up.....

If a boat was really bad, we used to get all the gelcoat below the waterline peeled off, then dried it out for a few months before repainting directly onto the exposed laminate. Fortunately the peeling was subcontracted to a company who had the right kit, I can recall doing one at the boatyard though. We landed the boat on a big tarp with some strops running right round , and then used a tirfor winch and some of the trees all around the boss' garden (I mean boatyard lawn) to careen the boats so the guys could get in and do the peeling. It's amazing the things we used to do with some bits of wood, bits of string, an occasional winch or jack and a lot of care.