Fish: I think I made a mess of my previous post. What I meant is that you roughly fair using colloidal silica filler after the first or second coat of epoxy. After that, let it cure for at least 24 hours, sand and continue with coats of epoxy (you'll never be able to do all 6 coats wet on wet anyway).
You can't do final fairing before all but one coat is on. If you do the rough fairing nicely, and considering it's below water line anyway, you may be able to skip final fairing completely.
About the blisters drying out. You must wash and wash and wash again for at least the coming week. Don't worry that you make it wet every time, it's much more important to wash it out.
Now "the plug": man I can't believe that. Well, grind it completely out and bevel it. The more round it is the better. You will end up with a hole and the bulkhead across it. A little more work but a stronger result so no worry.
This is how to build it up:
Start on the inside.Cut pieces of formica (or thin plywood) to close the hole on the inside. Overlap the hole 2 inches where possible. Just put them there so you can get them when you need them.
Next, the outside. You will need a piece of plastic foil and a somewhat smaller piece of formica. Both must fit over the hole incl. the beveled edge. You also need a work surface big enough or a couple of A-frames. Put the formica on it (nice side up) and drop the plastic over it so that it overlaps everywhere.
This is where the fun starts. Cut a piece of cloth so that it fits the hole and overlaps the complete beveled edge. Put it on the plastic foil on the table. Next, cut woven roving to fit but an inch smaller then the cloth. Put it on top of the cloth on the table, making sure that they are aligned top-to-top if you understand what I mean. Finish with another layer of cloth, again, an inch less overlap. Put it on the stack again.
Sit back, have a smoke and do some yoga. When ready, take the stack of three layers from the plastic and put them with the same orientation on a second clean surface. Take the first layer from that (the smallest) and put it back on the plastic. Mix a batch of epoxy and hardener (start with 3 "pumps" ; slow hardener if warm climate) and just pour it in the center of the cloth. Use a plastic spreader to work it outwards so that it completely wets out and air bubbles are worked out. The cloth must go transparent. Add more epoxy if needed, guess how many extra pumps you need. Move excess to the center. Now, drop the woven roving on top, center it so that it overlaps the first layer everywhere and smooth it out. Do the pouring and spreading again and repeat for last layer of cloth. On that last layer, you must remove excess epoxy that would sag otherwise.
Now you need more hands. Take a deep breath and put the whole thing, incl. plastic and formica onto the hole. Don't press too much in the middle. This is easier than it sounds although it is often done while cursing ;-) The helper must be experienced with electric
drill and screwdriver. He/she must drill holes through the formica into the hull very close to it's edge (1/4") all around every 2 inches and put small screws in. test for right size drillbit and screws into the hull laminate beforehand.
If that is secure you can breathe again but not for too long because there's more work on the inside. Start with looking and the spreader to estimate how many layers you need to add before you are flush with the inside of the hull. Use woven roving for all but the last layer. Work on each side of the bulkhead, don't try to get a layer behind it. Also, keep the fibers inside the hole, don't let them come onto the inside of the hull or the bulkhead. I would advise to do each layer apart here. Instead of pouring epoxy, you use a roller and/or brush. If the epoxy runs a lot, add a little colloidal silica filler but keep it as thin as possible or it won't wet out good enough. You can still use the spreader to smooth it out and work the air away to the edges.
When you are flush, cover it with plastic foil and screw the formica on.
After 24 hours you get the shakes and take the formica off. It should come off and also the plastic should come off. If not, don't pull too hard, even if that means you have to grind the formica away (can only happen when the plastic had holes/cuts) or sand the plastic away. Admire it and move inside. Here, you need to sand until it is dull. Also sand the bulkhead around the area of the hole. You know how to do fillets? You must put a fillet on the patch-to-bulkhead joint. Use a 50/50 mix of high density and colloidal silica fillers and make it a thick peanut butter consistency. Use a round-ended stick of about 1/2" diameter to scoop it on and shape it. Scrape excess away with flat stick or putty knife (use that further on for the fillets). When the fillets are on, start cutting 3" wide strips of woven roving to go over the fillets, creating a strong bond with the bulkhead. The fillets are just to create a minimum radius for the fibers. When the fillets are gelled but still tacky (doesn't take too long with the filler added and the thick layer), put some plastic wrap on a work surface, put a strip roving on and wet it out. Add some colloidal silica to the rest of the epoxy and slap the roving onto the fillet. Spreader, brush, roller etc. Now you were smart and got some 4" wide fiberglass tape. Cut pieces to length so that they are 2" longer than the strips of roving. Wet out and put it over the roving.
Now the finishing touch: cut fiberglass cloth to cover and overlap the hole and the joint (from one piece if possible) including overlapping the fibers on the joint. Wet out the dry part of the patch AND the cloth and put it on. Make it look extra good now. Let fully cure.
The finishing is sanding
and fairing and sanding
. After that is done, add two coats of epoxy both outside and inside.
If you have good access to that hull-to-bulkhead joint, examine it before the job and decide if you want to reinforce the whole joint instead. If it's in view, you can get a much better looking result when you glass the whole joint.
This is something that every cruiser who can sand and paint can do. Don't hesitate, the worst that can happen is to put the grinder to it and start over ;-)