I applied 'glass to the underside of the deck today. While it was easier that the cockpit, it still was a little fussy. I have no prior experience with fiberglass & resin work, and it seems like there is no substitute for experience here. I had done some experiments with fiberglass and wood while waiting for the kit to arrive, but it gets more difficult on a large scale.  It seems that fiberglassing is one of the major skills in wooden boat building, and I've had to learn by doing. I started at the bow, and got much better at it by the time I reached the stern!
If you are building a boat like this , here are some of my tips and observations.
LET the epoxy soak into the fiberglass.It takes some moments for it to saturate, and I had the most trouble when I did things to try & force it in.
Don't try to make the 'glass do radical things. It will NOT turn a 90% corner, for example. If you have to, cut relief darts at the tricky areas.
I used a plastic putty scraper to gently distribute glue over large areas, but the best tool is the lowly Cheapo Chip Brush. Foam brushes get soft & useless when saturated by epoxy. Use the chip brush to fill blank spots by jabbing at them with the bristles. I got into trouble when I tried smoothing bubbles with my fingers.
Try NOT to  work the fiberglass over & over in the same area. Causes bubbles, and little bits of 'glass begin to wear off & make a mess. If you overwork near the puttied joints, some of the wood flour will dissolve out, and make the clear epoxy  un-clear.
Do this, and anything else you haven't done before, on your day off, not before work or at night.
I found this a little stressful, and  I was glad that  I had "all day" to do it.