I think your problem is that you laid down that 20 ounce fabric as a first layer. I'm not sure why you even need anything that heavy for a deck. YOu might want to try putting on a thinner and lighter layer of glass cloth first. That won;t sag so much, and when it dries it will provide more support for the thicker layers (or additional layers)

Since your intention is to include maps as decoupage elements in your deck why not make the whole deck from paper first? Or, paper mache. Lay out thin strips or battens across the deck beams to support the flimsy stuff and grab a Sunday paper. Cover the battens with saran wrap or a cheap, thin polyethylene tarp. Pull this tauht and hold the edges to the hull with duct tape or masking tape to keep the plastic taut.

You apply paper mache made from sheets of wet and glued newsprint, and let it dry. If you put on a thick layer all at once it will take a long time for all the moisture to evaporate, so stop often, let the work dry, and then put on a little bit more.

Mix up a small amount of wallpaper paste with about 10% to 15% more water than the directions call for. Tear (don't cut -- the ragged torn edges blend better) a piece of newsprint about a foot square, soak it in plain water until it is limp Set that paper on the plastic which is stretching over your deck to be.

Add additional pieces of paper, overlapping the edges about an inch or less, until you have covered the deck. Now put on a second layer of newsprint, but instead of soaking this layer in plain water, paint it with, or soak it in, your diluted wallpaper paste.

By putting down an unpasted layer of paper first you eliminate having a layer of unsightly paste on the bottom. with no paste it will be easier to remove the underlying saran or polyethylene. Once the layers of papermache are dry you can easily peel off the plastic. The additional layers (with the paste on them) will bond to the top of that plain paper.

Apply two or three layers of this, then top with a layer of paper soaked in plain water. That covers any remaining paste. and gives you a neat top layer.

With all the overlapped edges you'll end up with a laminated paper deck form which is 5 to 10 pages thick.

If it starts to sag while you are applying the paper, stop and let the entire thing dry. That will make it stiffer and stronger. Then you can add the remaining layers.

When you are done you'll have a sturdy base which will support your first glass cloth layer.

If you would like, forget the top layer of paper which was soaked in plain water. After the paper mache dries you'll have some residue of dried paste on the surface of it. Paint a layer of wall paper paste on the back of your maps (you won't have to saturate them) and you can put them over the paper mache base. Let this dry and you'll be ready to put on the glass.

Apply a thin layer of glass fabric -- say 4 ounce or even 2 ounce to the surface of this. Once it is hard, put on another layer of glass which is twoce as thick. The combined base of papermache and your first layer of glass cloth should be strong enough to now support the heavier glass, and you'll have fewer drips and a lighter layup. Once the second layer of glass is hard you can lift the deck off the boat and pull away the supporting battens or strips, and peel off the plastic. Again, make sure the thing is bone dry, and then glass the inside. Once the first coat of epoxy has set you can put on a second coat (no need to sand) and then as soon as that second coat sets --while the epoxy is "green" -- you can remount it and trim it.

There are a few other options you an pursue, but the basic idea is to build thin, light layers first to redue sagging to a minimum. then you add more layers for the desired strength. By using a paper mache core you'll provide a base for your paper maps, and reduce the volume of epoxy resin needed. You'll end up with a composite, similar to that found in wood strip boats, with a sandwich of glass, wood fibers (the paper mache), and glass.

Hope this helps.

PGJ