Installing a new deck fill

[expozen]

I'm installing a new deck fill for my water tank. My deck is 1/4" fiberglass lam over 3/8" 3-plywood, over ~3/16" fiberglass lam. Then there is a 1/4" fiberglass liner below that. I usually use Dremel bit #115 to remove the core around hardware areas and refill with thickened epoxy. But what's the proper way to do something similar for a deck fill? The hole is 1-3/4". If I use the Dremel bit #115, it only removes about 1/8" of core around the perimeter of the hole. It seems like more material should be removed because of the size of the hole. I'm thinking of buying a 3/8" x 1/2" rabbeting router bit and modifying it so that it's flush on the bottom. If I remove 1/2" of the core around the hole, should I be adding something more substantial to the epoxy in addition to the usual colloidal silica (like cut up pieces of fiberglass)?

[Fore and Aft]

I never core out plywood decks, foam or balsa for sure but plywood is pretty easy to seal. If you are really worried you could glass the plywood edge with a bit of 6oz glass. I usually just wipe some epoxy resin on the plywood edges two or three times over the day and leave it at that.
Cheers

[Tillsbury]

I like the sound of your plan. Half an inch dug out, filled with very thick epoxy and either cut fibreglass or maybe even some offcuts of cloth stuffed in should be fine. You want to be confident when you tighten the bolts up to squeeze out the butyl.

[expozen]

I've decided to just use the Dremel bit #115. The router bit I was thinking about won't work because it's 1/2" high and cuts 3/8" deep, instead of 3/8" x 1/2" like I was thinking. And I can't seem to find any other tool to use. Since it's only 1/8" deep I'm going to use just the colloidal silica.

But I'm curious why it might be less important to seal a plywood core in the same manner a foam or balsa core would be sealed.

[Fore and Aft]

The main reasons I can think of is plywood is way less likely to rot as quick as balsa core. Foam will not rot but if you router it out then you have something solid to screw into when you put the deck filler down. When I mentioned wiping the raw plywood edges with epoxy resin by the third wipe at the end of the day no more resin should be penetrating the plywood and the last coat should be a sealer coat. But to be honest when I was boat building if it was a plywood deck you would give the plywood edges a couple of good wipes with epoxy resin then Sikaflex/Screw down the deck filler.
I am not sure using 1/8 glue ring is really going to achieve that much except for dragging the job out? People will argue that it seals the plywood, but then so does three good coats of resin. In fact the resin might be a better sealer since it has soaked into the plywood?
Cheers

[gulfstar37]

We had a similar situation. I was thinking of removing enough core where the mounting bolts/screws would be in the epoxy. I used a hex Allen wrench that had the one end cut off leaving a 1/4" chucked in a drill motor. It helps if the Allen wrench is cut on a angle, it cuts better. It's a little rough at first but as it cleans out it gets better. Another tool was made but leaving 3/8" as the cutting edge, then 1/2". The void was then filled with thickened epoxy. This should assure no issues of soggy core and gives the fasteners something to hold to without fear of crushing the deck.