So here we go for the recore (this is a bit of a long message, but..):
You will need a piece of 1/2" marine or exterior solid ply plywood for the core. Do not use treated wood or pine, as resin will not soak into and bond well with either.
You will also need enough fiberglass mat or cloth for 4-5 layers in the skin and 1-2 gallons of resin (depending on how you wet it out - more for mat and less for cloth).
Remove the rotted plywood core from the fiberglass skin with a wood chisel and/or putty knife. Be careful not to punch through the skin.
Carefully sand the sides and bottom of the skin to remove the ridges from the original core installation. Cut a piece of plywood so it fits easily inside the original skin - you don't want a tight fit - leave about 1/8" around the sides. Then drill 1/8" holes through the plywood on a 2" grid over the surface of the plywood. Use a table or other saw to cut a grid of 1/8" deep score cuts on a 6" grid over the side of the plywood that is going to be toward the skin. The holes and the score lines help the wood absorb resin and also prevent air pockets between the skin and the plywood.
Cut the mat/cloth to overlap the sides of the skin about 1" all around.
Mask the center hole on the finish side with some good masking tape and do the same with all the screw holes. This will keep some of the resin from ooozing out.
Turn the skin finish side down on a flat surface - you may want to put some wax paper or saran wrap down if you want to protect the surface - don't use newspaper or something absorbent. Then place 2 layers of cloth/mat in the skin shell and try to fold the ends over the edges a bit. Mix up a quart of resin, but go a little light on hardener if it is warm where you are working - you don't want the resin going solid too early!
Coat the scored skin side of the plywood with resin using a roller (I like a 4"). Then wet out the glass inside the skin shell with the roller and resin, wetting up over the edges. Mix more resin if you need it, and pour any leftover resin into the skin and smooth it out with the roller. Then put the plywood wet side down in the skin shell and put weight (bricks, blocks, railroad rails, the heavier the better) on the plywood to get the resin to ooze out of the drilled holes. I actually like to walk around on the plywood a bit to make sure... Resin should ooze out of all the holes. Use a putty knife or cloth to remove the resin that squirts out of the holes - it will make like easier later. As the resin sets, the holes will quit oozing. When that happens, you are done for the day - leave the skin overnight with the weight on it.
The next day you can take the weight off the plywood. Use a belt sander or grinder to remove any glass "tags" that would keep the next layers of glass from bonding with the edges or the plywood. Mix up another quart to half gallon (depending again on what kind of glass you are using) of resin. Place 2-3 layers of glass on the plywood and up over the edges of yesterday's glass. Wet the glass out with the roller and get rid of all air pockets. Note that this is the underside of the center panel. Once you are happy with the wet-out, leave it to set till the next day.
The next day, use a grinder or a sander to remove the excess glass/resin from around the edges of the skin. You want to get it as flush as you can, but try not to grind away the skin shell. You may see some voids or little air pockets around the edges - we will fix that later.
Turn the panel over and remove the masking tape. Any excess resin should come off pretty easily with a putty knife. Using the old hole in the skin as a template, cut the deck plate hole with a saber saw or cutoff saw. Fit a new deck plate in, caulk the skin with 5200, and put the deck plate screws in. Try not to get caulk down into the hole - just caulk the top where the deck plate meets the skin. Drill 1/4" holes for the screws in the same places as the old ones on the skin, and put masking tape over the screw holes on the skin side.
Turn the panel over again with the skin side down. Mix up a pint or so of resin and brush it around the edges to fill any voids or air holes. Also get some resin in the screw holes to waterproof and seal them.
Then carefully fill the space between the underside of the deck plate and the plywood with resin. This was Aquasport's problem when they made the boat - the wood around the deck plates was never sealed - and that's where the rot started and would eventially rot the whole panel. Get a nice resin soak on the plywood and area around the deck plate. If you want, you can put a final finish coat of resin over the bottom of the panel - but ain't nobody gonna see it once it's down. If you were really anal, you could even gelcoat or paint it...
Once everything is dry and set, you can put a big 'ol bead of 5200 around the inside of the tank tub and screw the panel down. Use a cloth with denatured alcohol to wipe off caulk that squirts up in the crack. Let the 5200 sit a day or so before caulking the top of the seam - 5200 shrinks a bit and you want to get the seam filled. Then caulk the seam and you're done!
