I don’t think the sort of bedding compound is that critical in this case when clamping down immovable deck hardware. Most stuff that is meant for the bathrooms, windows or cars should work. I have used anything from butyl-rubber (cheap and good), PL-400, silicone and polyurethane (like Sikaflex), and even plain rubber gasket sheet (1mm thick), and they all work.
It is more important to do it right:
1) Just clamp the hardware on loosely at first and leave it like that for about 24hours, then tighten the screws or bolts. That way one avoids squeezing out the compound. (that advice once gave me the idea of using pieces of rubber gasket sheet).
2) Make sure that the hardware is clamped on so tightly that there never will be any movement. If necessary stiffen the deck with a pad of plywood on the underside. Movement always leads to leaks.
3) I also seal the screwholes the best I can, and if I have the chance, I paint over the screw heads with 2-pot polyurethane paint. This has the ability to penetrate any gaps or cracks. On one boat (Albin Viggen) I had problems with leaks at the deck-to-hull joint. Painting over the hundreds of screws solved the problem entirely.
site contents © the Junk Rig Association and/or individual authors