When I bought my Merit 25 one of the issues was a soft deck. A huge chunk of it and I knew it was going to be a chore to replace. My decks were in very good shape and since I didn't have a liner (or interior) I decided to do the core replacement from the bottom instead of removing the much thicker fiberglass shell and gel coat from the top.
If I had a liner, or cabinets, or a painted deck I probably would've thought about going at it from the topside as this would've been easier due to gravity.
Let me also say that this was my first ever attempt at using fiberglass and epoxy.
I chose to use the West systems epoxy b/c it seemed pretty easy using the pumps (buy the pumps, the mix is too sensitive to eyeball, I know b/c I tried). I used 105 resin and 205 fast hardener along with 407 and 406 filler. None of which was cheap. I believe I used 6oz cloth for the layup along with 1/2" A/B plywood (marine grade). As it turns out, I should've used 3/8".
Removing the old rotted plywood was 1/2 the hard work. It's messing, wet, and labor intensive. There was a single layer of glass on the inside of the cabin. I used an angle grinder and a circular saw to make the initial cuts. Be careful when using a saw above your head (keep track of where the cord is, don't ask...). After making some "slash cuts, it was down to using brute force with hammers, screwdrivers and chisels. Then after the big chunks were out, I used an orbital sander to smooth out the underside of the outer shell of FG.
Used the same circular saw to cut my sheet of plywood into smaller "puzzle" pieces to start 'gluing' into place using thickened epoxy (406 silica). Remember to 'dry fit' the pieces first to save on materials. Remember the fast cure 205 crap hardens pretty quickly. I used 1"x1" wood cut to various lengths to wedge under the plywood to keep it in place while the epoxy kicked. Once it had (the next day) I'd come back and fill in any gaps using thickened epoxy and then glass over the “tile work” with fabric and UN-thickened epoxy as this creates a stronger finished product. Working above your head with epoxy of varying thickness is a messy task. Your clothes, face, gloves, hair on your arms, will be trashed, just plan accordingly (a drop cloth would’ve been a fantastic idea).
I then came back another day and sanded the semi finished stuff. Cleaned the surface and used 105, 205, and 407 filler to smooth out the texture of the glass fabric. Sanded that and let it cure completely. I then put 2 coats of Brightside white on it and let it be. I also had to replace an H support at the companionway opening due to the original one rotting out. I have attached pictures of that as well as the removal of the old wet core and the replacement. I have not taken pictures of the finished faired and painted cabin roof. I’ll try to get some soon.