Best practice for through bolts in cored platform

Stodge

I'm looking to switch dinghy and davits this Spring.  The davits I like use something like the Weaver slide in mounts attached to the platform.  These would be through bolted on the surface, not the indents.  This means making bigger holes that needed for the bolts, filling them with penetrating epoxy, then drilling the correct size holes in the epoxy.

I did this for my grill mount.  But I had a heck of a time keeping the bottom sealed so the epoxy would stay in and set.  I used duct tape on the bottom of the platform to close the holes.  But the surface is not smooth and it didn't take much of a gap to let the epoxy seep out.

Does anybody have a better method sealing the the bottom of holes to keep the epoxy in while it dries?

LaRea

What I've heard of is drilling an oversized hole only on the top side (not all the way through).  You can backfill with epoxy and it won't seep out.  

Another method involves bending a nail 90 degrees and putting it on an electric drill.  Then you drill a regular-size bolt hole, insert the nail in the hole, and use the drill to cut away the coring around the hole.

But I've never used either method.

Dude_Himself

I've also seen folks epoxy in a SCH 40 PVC tube slightly larger diameter than the bolt they intend to use - this keeps water out of the core and prevents some spiderweb cracking of the gelcoat under compression.