with a GRP washer you would have to grind away glass fibre to be able to match it with the hull before glassing in place. That would be a pretty messy process!
Thanks for the tips.
No actually you use a thickened epoxy or thickened fiberglass resin to peanut butter consistency. I thicken with CSM, Cabosil/Aerosil & milled fibers. Just a strong recipe that I like.
You then then slather the backing block in the peanut butter, and coat the hull too, and then bolt the, WAXED, seacock in place using the WAXED bolts to align it before the peanut butter begins to kick. Once the bolts are tight and everything is aligned make a nice fillet with your gloved finger going around the block with the peanut butter that squeezed out.
I use a slow epoxy hardner if the "gap" between the hull and backer block is not too thick. I don't want to much exothermic heat. If the hull is has to much curve I take some off the edges of he backing block, not the hull, then do the procedure outlined above.
When using fiberglass backing blocks I only use epoxy or polyester or vinylester to adhere the backer to the hull not Sikaflex of 3M.
If your design does not use a thru-hull a solid glass laminate is even more important than a plastic with Sikaflex. Morris Yachts does this same type of installation, through solid glass, but, with Marlon sea cocks and no thru-hull. They simply seal the exposed hole fibers with barrier coat or epoxy. It makes for a nice smooth hull but does leave you entirely dependent upon the bolts because you no longer have a thru-hull..