Oh lord!
Ok, your in Washington. You are sitting in boat building Mecca. Take advantage of your local resources. You have some of the best here!
First here's a good place to get your supplies : fisheriessupply <- wasn't allowed to post links but its close.
Here is another, AND they can give advice : fiberglasssupply <- wasn't allowed to post links but its close.
As for waterproof faring, if you started with polyester or vinyl ester you want to use 3M Marine Premium Filler. Its like Bondo but permanent and waterproof. Bondo will suck up water and die after awhile.
IF you started with expoxy, you need to stay with epoxy. You can make fine faring compounds with epoxy they are just harder to work with. At the last minute, just before epoxy sets up, it goes through a runny stage causing you're perfect fillets to sag.
Don't start randomly mixing chemical systems. Down that path madness lurks. Most of these chemicals systems don't get along.
Are you trying to protect from water spraying on the inside or from water getting in from the outside? From the pix I'm going to guess inside.
If I had to seal up the inside using polyester? I'd..
Paint everything with a coat of the resin to seal up the wood and give something for the rest of my work to attach to.
Sweep the radiuses with the 3M putty. (Assuming you don't need to tab the seams. Do you need to tab the seams?)
Do a rough sanding with the longest sanding blocks possible. Maybe to 80 or 60 grit? Certainly no finer. This is rough shaping.
Re-coat everything with resin one last time.
Now skincoat the entire inner surface with.. 1oz mat. (Remember to get ALL the air out!)
Lightly sand the prickles off once this kicks off. You should be able to run your hands over everything without getting stuck or cut.
Get a long flexible plastic squeegee and coat the inside with the fairing putty. This will be a thin layer.
Sand smooth to 80 grit only.
Take your gelcoat, add wax (surfacing agent) and spray it in there. (Good luck getting that step to work well.)
When it kicks off, your done!
If everything is done right you will end up with a smooth Matt finish. This is used a lot for racing sailboats to finish off the inside of the cabins. Racing sailboats need VERY light interiors that can take a beating.
Hope this helps!
-jim lee