Originally Posted by The Gurnard
Its your way.. and it could be considered the correct way..but why ? ..do you think this method will fail ?
I hope Im not sounding arrogant by asking as its not my intention..Im just wondering why ..if I get a leak..do I have to make another to repair the first..then have to repair the one I just made..if that makes sense ?
Not at all!
Glue is very weak. It is probably one of the worst ways to assemble something.
In order to maximize chances of success, you need to pave a road to success. Your method may work in the short term, but there are a lot of variables. You are mixing two different chemicals together which could yield mixed results... they operate by two completely different curing mechanisms.
PVC boat glue is nothing more than ~25% solids suspended in a lot of solvent. Typically, it is some kind of synthetic resin It is applied to the material, then, the solvents evaporate. This drives the solids in to the material, enhancing the bond strength. The problem is, is that if it is applied too thickly, it will skin over. The solids at the surface will dry, will the solids at the bottom will remain wet because the solvent is trapped with no way to escape. Then, when the part is laid up, the surface skin will break, releasing the solvents, which will then deactivate the bond you just tried to make.
Mixing with sikaflex may entrain solvents, which may weaken the bonding of the PVC glue, of the Sikaflex, or both.
Ultimately, if the repair fails, you'll be stuck removing that globby mess as well which may permanently damage the material.
By going from the inside of the boat, it allows you to:
Apply thin, even coats of glue to both the patch and the boat
It allows you to maximize your patch overlap, which will increase the holding strength of the patch.
Best of all, it is a known working method! By applying a patch from the inside, it also makes for a stronger patch than from the outside. Instead of pushing UP the patch, trying to make it bulge, instead the air pressure is trying to push that big patch through the tiny hole. The chances of the patch peeling free ever are non-existant. Since the patch is on the inside, it is no longer affected by chemical exposure, UV, and there's no mechanical wearing motion to possible cause an edge of the patch to lift.
My 2 quid. Take it as you please. I understand why you're hesitant. For the first 2 years when I apprenticed fixing inflatables, I tried to avoid doing inside outside patches at all costs because I was very bad at them...