Re-read your message, went to edit my post and I'd written longer than 30 minutes..
Here is what I would do:
To my eye it looks like you've got a layer of polyester over the deck, the grey fiberglass, and a layer of epoxy over the top... the yellow.
The epoxy turns yellow after 2 years or so in the sun, and has zero strength and is almost like chalk.
Polyester over wet wood doesn't adhere.
It looks like you've got two layers of 10 ounce finish cloth, ideally when you go back use a layer or two of 17 ounce biax so you have enough meat over the wood that if someone drops an anchor
or hammer, it doesn't star crack and let water
back in to rot
it back out.
Is the existing deck two layers of 1/4?
1/4 is actually fairly tough to get a good lamination over framing, as you can't put but some much weight on it before it deforms... and you can't really screw laminate it either unless you run a bunch of strips on the underside. You'll use about 1/2 gallon of epoxy per 4x8 sheet to laminate it, on the dry side. If you mix it to thick, you'll get pockets where the epoxy didn't move out from between the layers. If you mix it to thin, you'll get dry spots. Dry spots are bad as two 1/4 inch pieces of plywood held apart by air sound like a drum skin when you rub your fingernails across them. About the only way to repair that is with a grinder and take off the top skin.
If the finished deck needs to be half inch, Use 1/2 inch plywood.
Doing a bit of scarfing work... or even doubling blocks between the framing will make your life much easier and it will cost a lot less, as it won't take nearly as much epoxy.
Lay some battens across the frames once they are exposed, and glue 1/8th inch marine plywood shims to any low frames, rather than pulling a pucker into the new deck you can fair the framework in once. Screw a 2x6 to the underside of the deck down the centerline and put a stiff leg under the front of the cabin
coach roof. She will sag, without her deck.
The biggest time saver, is to dry fit the plywood and use the same screw holes to set it back in place... That way you can go inside with a pencil and mark them. A needle nose vice grip with a 3 inch stub of pencil works well for corners you can't quite reach. pre-paint the underside of the deck after marking for the framing... I like to use epoxy paint
, and leave the wood exposed. Resin coating is grand, but not only is it an extra step... its a mechanical bond rather than a chemical.
When you go to laminate on the plywood, tape along the paint lines leaving only the sanded plywood left to be glued. Tape up the sides of the frames... Then when you butter up the frames and set the plywood down into place, and screw it down, go under and wipe off the excess, then pull the tape lines 10-15 minutes later. When the epoxy kicks, pull a finger fillet of your favorite paintable caulk.
overhead, no painting overhead.
After the deck is laminated in place...
the seams down an 1/8th of an inch on the top surface of the deck at each butt joint, if you use butts. If you scarf it, grab a long board and long board sand the top of each scarf until they lay down flat. Then lay a piece of tabbing across it. Do the same at the sides of the house, if they are still in good shape. Tab the deck in to the house, then lay the fiberglass up into the fillet but no further. It makes for a lot cleaner job, and a lot less sanding
if you make the tab a low spot that can be covered with putty instead of re-fairing the cabin
sides. Same goes for the hull
to deck joint. Wrap it with tabbing, and don't try to do it with one piece of glass from the cabin sides to the topsides. You can do all the glass work in one day, or all at the same time... Just don't punish yourself if you can avoid it. Grin.
If you have to stop for the day, put peel ply on the last piece of cloth that will accept the next layer tomorrow. As a general rule
, I prefer using finish cloth to peel ply... but I do use it to save having to amine wash, right beside raw plywood. Water and plywood that is to be glassed the same day don't mix very well. You can use denatured alcohol for the wash down instead of water.
I use a west systems black stir stick for glassing corner fillets. They are big enough to let the cloth lay in smooth. Cabosil works well, with a touch of chopped glass fiber. I pull these fillets wet, where right before the fiberglass goes on, the fillet is pulled and cleaned up. Work the air roller in to the fillet and any get no bumps.
Use tropical hardener and do it in the shade. It makes for a lot less waste, as everything is less likely to kick off on you. Don't over work the cloth, once it lays down true move on to the next piece. biax will unravel at the edges if you work it to hard. This is a good thing to remember, a gloved hand and a quick rub will unlay the fibers enough to work a wrinkle out.
17 ounce biax without a mat backer is a little tougher to work with but takes a lot less resin to wet out, saving a bit of money
and weight. Wet the plywood with unthickened resin, until it will absorb no more. Normally takes 2 ounces per square foot, though sometimes the grain is a bit more open and it wants more.
If you have the deck fair before the plywood goes down, and the seams already taped and ground smooth, the fairing work after fiberglassing should go very quickly. I normally lay a layer of 10 ounce finish cloth over the wide open spaces and a 4-6 inch wide piece of finish cloth over the tabbing, and use a 6 inch air roller to work things smooth and fill the weave of the biax.
I want to see the weave of the biax after air rolling, which is a good indication that the layup
isn't overly wet. Wet layups, the fiberglass will actually float on top of the resin and make high spots. Wet it until it goes clear. I use a 9 inch paint roller, in a white plastic roller tray. 3/8ths or 1/2 inch nap works well... You waste a little resin wetting out all that nap, but its what i use.
Once you are satisfied that you don't have any dry spots, and aren't to wet... (Lift up any excess with a plastic squeegee, and spread it around) let your work begin to gel.
Now roll on a layer of epoxy, and lay some 10 ounce finish cloth into it. I roll the layer of resin first, in the event any localized spots have gotten extra sticky, and to fill the weave of the biax so air doesn't bubble through the finish cloth. Air roll the resin through the finish cloth and work any excess over to the dry spots.
Finish cloth is the cheapest fairing material there is...
Once it starts to gel, roll a layer of neat epoxy over the top, and squeegee it out so you can't see the weave of the cloth.
The next day, wipe down for amine blush, and sand it out until the shine is gone with a DA sander. 40 grit is fine... Don't sand through the finish cloth unless its just a bump the size of a quarter. Lay the same battens down on deck you faired the frames with.
I normally use an aluminum
yard stick laid flat on deck. Mark out any air gaps, which are low spots... fill them with a 12 inch sheet rock knife. Finger sand any small low spot the size of a quarter, and circle them with a pencil.
Start in the corner of the cabin house and pull toward the toe rail. If you did your job fairing the first time, it'll take a quart of putty to fill the whole deck. Don't blob it on... 1/8th inch thick at the most. If you laid your fiberglass butting between layers instead of overlapping there shouldn't be any high spots to make a low spot that will accept any more than that.
Once the deck and cabin corner are straight, flat, fair and true, pull a decorative fillet around them with putty. I normally use a 1 1/2 pvc pipe. A sharp corner looks good on older boats... but you have to have a fillet of some degree to keep the paint stuck in the corners. 1 1/2 is the smallest that is easy to sand. Don't sand the fillet any more than it takes to take the shine off with your finger tips, if there are any sharp points sand them off.
Some folks float the whole deck in putty, then sand it back out the next day. If the work is fair, and the sander doesn't jump all over the place I prefer to resin coat, and in the polyester world... gelcoat
... as it lays out an even coat, and builds the whole surface out. On flat stuff like decks, it is just so easy to pull a high spot in and not sand it all the way out... then spend the next few days filling around it until you've puttied the whole deck, I don't really see the need. Fill the low spots, once everything is level resin coat it to bury the sanding scratches.
I like to resin coat everything 3 or 4 times, on the last day after I'm pleased everything is flat. Treat the resin coating as a high build primer that fills 40 grit sanding scratches. Sand out with 80 grit.