: You are absolutely correct, and that is my issue. My deck is
: designed to be flat where I have it mounted but I have about
: 1/16" of camber (1/32 on either side of the centerline),
: which is enough to keep it from sealing correctly. It was flat
: before I joined hull and deck, but I picked up the camber when
: fitting the two but since I was already fiberglassed I didn't
: want to tear into it to flatten it again. I have tried
: everything I can at this point because I have a batten to
: reinforce the flat deck just aft of the hatch that I would have
: to cut through or otherwise modify and I have been reluctant to
: do that.

Mike-
Another approach to this problem (you are not far from dead flat @ 1/16") is (starting with a clean deck and hatch rim) to put a few (fingernail-size) shims on the deck to hold the hatch rim flat. (Glue the shims in place on the deck-make sure they will be covered completely with your sealant/adhesive).
You can even glue the shims in place and then sand flat with a piece of (flat) plywood with sandpaper attached.
Apply sealant/adhesive and bolt/screw/weight the hatch rim in place with minimal pressure. The shims will keep the hatch rim from flexing. Clean up excess adhesive.
Let the adhesive/sealant cure well (days to weeks, depending on the goop) then tighten up the fasteners.

I used this method for making epoxy pads for stanchion bases on a sailboat project, and I've also used it with inspection ports & hatches with 3M5200 and Sikaflex.

BTW, I've also been surprised at how flexible glass/CF layups are- it takes quite a bit of glass/CF to make a stiff flat panel.

Cheers
John