Originally Posted by Streetcar
...This is a big hatch and will be held in place by self tapping screws into GRP and adhesive/sealant. I am worried that using an adhesive/sealant like 3M 4200 will not be structural enough given the slight flexing in a GRP deck. Should I consider using a polyurethane 5200 type bonding sealant?
You are right. If you are using self-tapping screws into GRP you better use epoxy
on the screws and 4200 under the frame - do not use butyl. If you can thru-bolt with a nut under the deck, then you are not only better off, but then you wouldn't need to rely on the "glue" to keep the hatch in place.