Ubergeekian
Well-Known Member
http://www.boatbuilding.net/article.pl?sid=06/02/14/142213 said:
You'll spot the word "cured" in there, of course. There is no doubt that if you have reasonably recent - ie not fully cured - polyester, then the best thing to put on top of it is more polyester, since the two will bond chemically to produce, effectively, one big cured lump. When it's older stuff, like a well cured boat hull, you don't get that effect, and you rely purely on the "stickiness" of whatever you're putting on top.
Now, I had two problems to deal with. First of all, the new material had to go on top of an existing hull. It doesn't make much difference what you use for that: the interface will always be the weak point. Whether it's unacceptably weak - it isn't - is another matter.
Second, I had the stainless keels to embed. For that bit of the job, a high strength structural epoxy was clearly indicated. On application that's a thick, black evil stuff, quite unlike layup epoxy. It also sticks very well to both old and new polyester.
Finally, there were some cost implications. The repair took around five gallons of polyester resin - doing it in layup epoxy would have been much more expensive for negligible benefit.
So there we have the constraints. And here, in detail, is the bonding sequence.
First, clean up the hull inside, first with a high speed rotary brush then with some careful grinding
Next, fill up the "grooves" alongside the keel mounting. That's effectively just a bit of filling, about 2" wide by 1" deep, so polyester was fine. Foam with a skim of glass, or even just some balsa, would probably have done just as well but the polyester was on hand and it didn;t take long.
Next, prepare for fitting the floors. Cut grooves/channels as described in the now slightly built up mounting, and carefully bend the floors individually to shape. Possibly the fiddliest bit of the whole operation, consisting of one of us bending them in a big vice beside the boat and the other testing them in situ: "needs to come up a bit at the end", "bit curvier there" and so on.
Next, bed in the floors, using high-strength (and high-shear, iirc) structural epoxy. Some of that bonding was to the new layup in the middle, but most was directly onto the prepared inner surface of the hull. Epoxy is definitely better for that than polyester.
OK, so far so good. That was one aim of the job well under way - strengthening the keel-hull join and spreading the load. The other related aim was to add some strength to the hull and that was done by building up the layup considerably over the new floors and beyond. What was exposed at this stage was a little new polyester, a fair bit of stainless and a lot of old polyester, since the layup had to extend well beyond the floors.
The solution arrived at, after much discussion, was to put another layer of structural epoxy on top of the floors, then build up the whole thing with polyester. The stainless-epoxy-polyester system was much stronger than straight stainless-polyester would have been. Structural epoxy for the whole thing was out of the question, because it's gloopy (technical term) and not remotely suitable for layup. A layup epoxy would certainly have been a bit stronger, but the interface with the old hull polyester would always have been the weak spot and the new polyester was quite adequate and cost effective.
Would I do it the same way now? Yes, probably, subject always to noting that polymer chemistry and the market place have both developed over 20+ years. I would certainly think about using an epoxy layup, but that would need a good cost-benefit analysis. I'm very happy with how the job worked out and I have been very happy to trust my life to the results many, many times since!
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