Certainly get the book Denny is recommending. But with that
recommendation, it is implied that you should go with epoxy. I'm all
for that too.

Yes, you're correct that epoxy would make a much stabler base, but you
would still have to coat it with a lacquer with UV filters in it.
However, that will last longer because it is on top of epoxy, and the
epoxy will last longer underneath it, because of the lacquer.

Without having seen it, I'm not 100 percent sure why you would waant
fibreglass on there - except to give it some strength. If the hatch
doesnt flex in any way, and you think that that is the reason for the
cracking, there is no need to use fibreglass at all. Imean, if it's
just because the different woods expands at different ratios, you
wouldn't need to have fg in there.

On another note, though, you will want to make sure the hatch is dry
(as in preferably down to 13 percent moisture), and you will also want
to cover the back of the hatch with epoxy. Not just one surface.

Basically, I'm not really feeling greeat recommending the mastodont of
west system, but it's everywhere, so you should get their ordinary
resing, preferably with the 207 hardener, but you can do with the
ordinary 205 hardener (cheaper without UV filters), and use a good
varnish with plenty of UV-filters on top of that.

About the fiberglass hatch. You will need to remove all the porous
stuff, all the degraded stuff from it. And if it still has enough
strength, you can do and use the exact same approach as with the other,
but for the extra strength get a thin fg woven rowing190grams/sqm, for
instance, also because in that thickness it will be see through.

After sanding the hatch down, removing the degraded stuff, fill it with
epoxy (it's pretty thin when it's non-filled), make sure it has enough
of it (think of it as "end grain wood), then put anothe layer of epoxy
wherein to lie the cloth, and then when the cloth is on, add more
resin, filling the cloth, removing excess (don't put too much pressure
here, as you _will_ stretch the cloth), and then when that is dry
enough you will be able to see where the roving interweaves. Then after
it has dried enough (not completely), you add the final coat of epoxy
to make it entirely smooth. Put varnish with UV filtersover that too.
I think a mat varnish would be better here.