If you need to seal the hole on the inside use a seriously sticky tape like duct tape. Masking or other types of tape probably won't stay stuck in place and the leaked epoxy is a mess to clean up.
Don't like drilling a larger hole. You substitute a continuous FRP surface for a lump of thickened epoxy with limited bonding area to the surrounding material.
On most boats the cabin
sides are solid glass. Leaking ports
are a major PITA but are not the cause of your wet deck core. If the core is just wet in a small area vice rotten/decomposed in a large area, filling with epoxy through holes drilled a few inches apart will take any sponginess out of the deck.
If you want to do a permanent fix for any fastener through a cored deck do this.
1. Drill holes through the outer laminate only, if possible. You don't want to go through the interior
laminate if you can help it because you'll have to seal the underside to prevent the epoxy from leaking out. If you drill through the inner laminate or dealing with an old fitting, you'll have to seal the underside of the deck. Use duct tape or other seriously sticky tape to seal the hole. Don't even think about using masking tape and you know how I learned that. If there is a liner, best thing is use a suitable hole saw and cut a large enough hole that you can use tape to seal the inner laminate closed. Fill the hole you’ve cut in the liner with trim plugs available from McMaster Carr or others https://www.mcmaster.com/#snap-in-plugs/=1a8svq4
2. Get a Dremel tool with a Dremel 199 bit.
Cut the Dremel bit into the hole at as close to a 90 degree angle as you can. This makes for a very minimal enlargement of the drilled puka to maintain deck integrity and in most cases the fitting will cover any damage to the gelcoat
. Once the bit is cut in, raise the tool to vertical and rout out the core. I’ve tried the bent nail, sharpened Allen wrench, etc, without a lot of success and a nearly broken wrist. The Dremel 199 bit works way better, less damage to the gel coat and easy to do for fastener pukas. If you’ve other than just s fastener puka, the other tools might work
better for routing larger areas though not for me.
3. Once that's done, vacuum out the hole. Fill a syringe with epoxy resin and fill the hole. This soaks the resin into all the void to be sure the core is completely sealed. Have discovered that West Systems has a very slow catalyst (#209) that will allow you to fill a bunch of pukas without the resin kicking in temps over 80 degrees. Use the regular slow hardener (#206) if temp is much below 80 degrees or the epoxy will take forever to go off. If you need the fast catalyst (#205) it’s too cold to be working and wait for spring.
4. Suck out as much of the resin as you can and mix with a structural filler like West 404. Reinject the thickened epoxy into the puka.
5. After the resin has set redrill the hole. The thickened epoxy makes an incompressible base for the fasteners. Personally believe that most of the leaks
into deck core is because the installer puts too much torque on the fastener and compresses the core.
6. Finish by chamfering the edge of the hole with a counter sink bit. The chamfer allows for a thick donut of whatever sealant
you use around the fastener shaft. That goes a long way to insuring that the fastener won’t leak again. Whether you decided to do the epoxy thing or not, chamfering the edge of the fastener puka is a must for any hope of a leak free install of any fastener.