One problem with the hold-down screws used to fasten the hatch is that they're basically sheet metal screws which have very sharp threads. Since virtually everything on a boat moves just a little bit when you go pounding into some big waves. those sharp threads actually cut into the fiberglass and eventually come loose. I've had much better luck using machine screws with threads that aren't so sharp. If you make a small cut in the threads with a dremel tool cutoff wheel, that'll make 'em self tapping. I'd suggest filling the screw holes in the lip that the hatch sits on with some Marine-Tex, leaving a blob of material on the bottom, then once it cures, drill pilot holes slightly smaller than the screw. I think a No. 10 screw will be about the right size.

You'll probably want to caulk the gap around the tank hatch to keep water off the tank, but DON'T use silicone caulk because if you ever want to paint the deck, paint won't stick to any area with silicone on it, and you'll never be able to remove the silicone! I'd use something like West Marine's Muli-Caulk.. It's also a good idea to lay some 40-50 lb test monofiliment down in the gap with a small loop sticking up before you caulk it. That way, if you ever have to pull the hatch, just pull up the mono to cut through the caulk!

Good ideas, thanks Bushwacker! My main concern is screwing two things together with a major air gap in the middle. The screws won't really be "tight". You're right about them being sharp deep threads. On one, the hole size had expanded to basically the size of the screw head, and was loose before I started the repairs...actually both middle ones were loose. I'll be checking out the health of the threads before I drop the hatch in.

Still need to drill and countersink the holes back out:
http://i304.photobucket.com/albums/n...ps0qtmofms.jpg

How is removal of the WM multi-cauck (or 3M 4000 UV)?
What an absolute rip-off, but I wouldn't expect anything less from WM!

I chose a different approach.
I filled the hatch screw holes w marine tex. Even with the longer hatch of a cc, the coffin cover is not structural. It just sits there. Fasteners just invite ingress. Sealed with life caulk and 80 lb mono tacked to the bottom for the zipper.

Seal the deck oval where the riggin` goes down w epoxy on a cc.
That tank looks far worse than my `89 original on top.

This is one of many reasons why I love CSC.
We love. We share. We learn.
Cheers,
GFS

So this project is complete and I'd like to thank everyone for there input answering my questions along the way and offering some much needed advise and guidance!

So I drilled out the existing holes in the hatch cover and re-countersinked them where needed. I then spot filled some dings w/ gel coat while I had the hatch off. I noticed that the original arch in the plywood translated to a now-rigid and visually arched (front-to-back) hatch cover :(. This made me very nervous, but turned out to a non-issue, as the two center screws pulled the hatch flush into position with ease and/or snugging them with my bodyweight on the hatch...it's rock-solid, but compliant.

So I cleaned the flange to a reasonable degree, and while the hatch was open, I cleaned up the tank with a wire brush and also cleaned the electrical contacts on the tank while I was at it. That funky oxidation appearing earlier in my thread was just
surface deep and cleaned up with relative ease:
http://i304.photobucket.com/albums/n...pshkrt56vr.jpg

I took bushwacker's and gofastsandman's advice and used the WM multi caulk, depite the trepidation of spending $30 on a tube of goop. I also bought and installed 40 lbs monofiliment as a "zipper". This took a while, as it was frustrating keeping it down in the crack all the way around where it needed to be. I jammed some blue tape down in a few spots and taped the starting loop end so as not to move.
http://i304.photobucket.com/albums/n...pspaeqxtrv.jpg

I repolished the screw heads, checked the health of the threads (all good!) and sealed her up!
http://i304.photobucket.com/albums/n...psosfsva6w.jpg

This was the next day so those specs of tree gunk, bugs ect sprayed off of the caulk, thankfully! Please pardon the appearance of the rest of the interior; It will be cleaned up soon! I was able to move the boat out from under some trees yesterday, wash the upper/outer deck and hull and start compounding her. The interior cleanup will be next.
http://i304.photobucket.com/albums/n...psuooualjv.jpg