Drill-Fill-Drill's done to protect end-grain layers of ply from water intrusion. The edge grain gets a coat as well as a plus, but it's the end grain that benefits most. So thickened is often unnecessary, unless you're doing something that requires more epoxy than just protecting the ply edge like bonding fasteners, a more structural approach.

How much oversize? Depends... sometimes no oversize is needed (really small fasteners) otherwise 1/32" all'round's plenty. For 1/4" hole that'd be 5/16" oversize, etc.

Tricky prospect is drilling the final hole on the same axis as the first one, so some means of maintaining where centerpoint is needs to be considered. I use packing tape-covered cardboard for a hinged registration guide myself, takes the uncertainty out of locationg that hole.

Yes, tape is used on backside to prevent epoxy from getting out of controlled placement. I'll even use disposable syringes (w/ or w/o needle) for more precise placement where & when indicated for a cleaner result & epoxy conservation.

Gougeon Brothers literally wrote the book:

https://www.westsystem.com/instruction-2/epoxy-basics/bonding-fasteners-hardware/

More than you may need to know for your project but at the same time a good resource for knowledge about epoxy, wood/plywood & hardware mounting.

   spclark,

Good info, very helpful.  Thank you.

Tim

It sounds like you are already on your way on this, but I thought it was still worth sharing a tip I picked up elsewhere on this forum (not sure which post though). If you keep the axis of the hole very perpendicular to the ground when doing the "fill" step (so gravity keeps the epoxy evenly pooled in the center of the hole) a slightly concave surface will likely form as the epoxy settles and cures. This creates a serendipitous guide for your bit in the final "drill" step that keeps the final hole nicely centered. To achieve this, I had to do my fills in several steps, repositioning the boat each time to get gravity pulling just the right way.

I also did some tests on a piece of scrap wood and found that thickening the epoxy with a little wood flour helped make the "fill" step more manageable, but I'm sure everyone finds their own recipe here.

That gravity-assist tip is a useful one, I've done it myself.

On not-so-horizontal surfaces one can back both sides with transparent packing tape then use a small disposable syringe fitted with a 16 ga. needle to poke thru one of the tape pieces to fill the hole to the max. Not much leaks out, the hole is well-saturated with unthickened epoxy, which is the point after all.

Add a bit of wood flour if you must but be sure the mix is liquid enough to saturate the end-grain bits of the plywood. That's why this procedure is recommended after all, and there's no reason at all you can't back up the unthickened epoxy step with another using your-filler-of-choice (powdered aluminum works great for anchoring threaded fasteners that need a tapped bore to work in!) to create a durable fastener location.

The link in my previous post offers good info on what kind of research has been done on epoxy-enhancing fastener locations in ply & solid wood. Another one's what Mike Waters has up on his website: https://smalltridesign.com/Trimaran-Articles/Construction-tips/mounting-hardware.html