The way the system works is to encase the wood in glass and epoxy.  So you encase the hatch just like the rest of the boat. Same with the hatch sill. 

You wrote, "Should I brush that on coating the underside after it's installed? Or should some of that be covered before installing it?"

You should install , glue it, the hatch sill with epoxy  there is a lot of stress there sometimes. AND also cover with epoxy top and bottom  once installed.  Often there is some deck shape the sill needs to conform to so you want it to glue and take that deck shape. 

You should not install the hatch with wet epoxy under it.  Unless you don't want it to work. But it needs to be epoxied and varnished top and bottom. 

   This blog is very informative for users, thanks for sharing!!

   Ok, so glass inside and outside of hatch. Thanks. 

   Hello, So if the idea is to glass the inside of hatch as well how do you deal with the edge of the hatch doubler. The glass won't form around a sharp edge and I believe a fillet will interfer with the rim on the sill. Am I missing something?

Thanks

Tom

   Dont confuse epoxy coverage as a sealant and epoxy saturated fiberglass cloth for strength. Coat everything not alive with epoxy to seal out moisture. Use fiberglass cloth and tape per the manual for strength. The WD14 has a rear hatch just behind the cockpit. Exactly where you sit when getting in the boat on a beach. The manual recommends adding a cloth patch for strength. Both hatches need fiberglass cloth on both sides and need to conform to the deck shape at the hatches. Enjoy the build.