[Ed]

I would say 40% of the hatches I did were done with marine ply where weight was not an issue and/or cost was a factor. Rich hit the nail on the head for installation technique

The remaining 60% were done with Divinycell on some very large hatches. Same basic process, except that after I laid down a new layer of glass (and before I installed the Divinycell core) I troweled out some Divilette onto the fresh glass. The Divilette is a pink liquidy-putty that acts as bonding adhesive (same sort of idea when you lay tile down on a bathroom floor) After that two layers of biaxial were laid down and then I trimmed around the edges when the glass was still green so I did not have to do any grinding afterwards.