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How can I get a smooth edge on acrylic and drill for screw holes w/o worrying about the sheet cracking? Any special techniques? Thanks.
Dow 795 and Dow 791 are both silicon based sealents, so if you are planning to seal the acrylic against the fiberglass you may want to consider a different adhesive. As has been discussed here, silicon is bad for fiberglass.Have you considered using something like 3M VHB tape, as an adhesive, instaed of drilling / using screws / bolts that will leak later. Also use something like Dow Corning 795 or 791 adhesive sealant to keep the water out.
Here is all the data:3M™ Scotch-Seal™ Polyurethane Sealant 540 and 3M™ Scotch-Seal™ Polyurethane Adhesive Sealant 560 are one component, moisture curing products which form permanent elastic bonds. They bond to a wide variety of materials including plastics, FRP, SMC, aluminum, steel, coated metal, and wood.
I misspoke (mistyped). The issue with silicon on gel is that (according to the books and threads I have read) gel does get porous, and the silicon gets in there and is very hard to get out.... but I have never read that it would harm the fiberglass / gelcoat.
Here is the data sheet for Dow 795:Though DC 795 is silicon based, it is not of the "vinegar smelling" variety, which, I believe, is the "problem" silicon to use on boats in general.
We have done several boats with it, and years later it still stands up OK and has not leaked.