When waterproofing , More is better !!! ^ Can’t argue with a boat builder ...
When using “the mix" we apply as many coats as possible , or until the wood will no longer accept any more ... just pools on the surface ... Thinking the “theoretical ideal “ would to be to totally encapsulate every fiber of the wood , eliminating any chance of water intrusion ...or the dreaded Rot ...
Fiberglass can be laid over most anything (within reason ) as long as the underlying surface is “prepped" properly , which usually involves much sanding and cleaning ... For trailer building , I could see/understand waterproofing under fiberglass , otherwise , if the glass cracks , the literal “sponge" of untreated wood anxiously awaits the water intrusion ( we’ve seen much of that on older FG boat transoms ) ... You would have to wait for the first surface preparation to totally cure , and then re-sand (tooth) and de-grease , before applying the glass ... Lots of work , Yes , but then you’re ... “protected” ...
Ps.
It’s always BEST to use fully compatible products ... read the can ...