Its doable,but gelcoat(mainlypolyester) don't stick that good on epoxy. For a deep chip like 4 mm, would fill it in 2 stages. First 3mm gelcoat mixed with what ever makes it thick. Then when its cured, another layer of gelcoat on top covered with film. It can be done without but i rather use the film.
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fun on a boat is inversely proportional to size...sort of anyway
That's what I've been thinking but wasn't sure how well gelcoat adheres to gelcoat. When repairing motorbike fairings in my youth it was always in one coat- 2mm max- with cling film over, direct to the fibreglass/resin. It's been the thickness and the fact it will be applied upside down that's been concerning me.
Appreciate all the tips.
Gel will stick to gel. Don't film coat it and don't use flow coat for the lower layers if possible. You'll end up with a tacky surface that the second layer of gel will stick too. Its shrinkage that causes issues. Just be aware that 6mm of gel coat has about as much structural integrity as a wet paper bag so while it will look solid if you hit it hard enough it will collapse. So don't use it to patch holes. What you are doing sounds fine.
gtflash - the gelcoat chips I'm filling are on the chines so are quite thick/deep. They go to the fibreglass beneath but not in to it. The gelcoat is more than 2mm - more like 4mm
If the gelcoat I'm filling is more than a couple of mm, why would you not fill with the same material? Would you be filling up the chip to 2mm shy of the surface? If so, why, and with what material? Thanks
Apologies. Damn predictive text. As others have said more than a couple of mm gelcoat is likely to crack. Gel alone is far too brittle and no strength. Use a structural filler like gelcoat mixed with cabosil or ready made filler from ecs. I struggle with a perfect surface finish with this filler but it might just be my method. I usually end up filling fine pin prick holes with gelcoat or a smoother filler. Good news is it also doesn't need wax and is very strong. I'm sure it used to say up to 5mm thick on their web site.
