Step 1: I Hate Raw Fibreglass
First thing you need to do is wash it all really good. Like. Really good. Grease, rust and other smeg will really roger everything else up.
Step 2: Tape Up Everything You Don't Want to Coat
1) DONT USE BLUE PAINTERS TAPE.
It sucks. It doesn't stick and let's paint creep under. You can use something like frog tape... Which actually works, or just use good 3m tape. Make sure it's a new roll. Not something you've got laying around. Also. Make sure it's got plastic on it when you buy it from the store. Air is tape's #1 enemy.
2) buy the green tinted paper
It will keep paint from seeping through the pores of the paper and staining whatever you've masked.
3) wear a mask. Not a dust mask. A carbon filtered painting mask. This paint I'm gonna tell you to use is not friendly.
(As you may be able to tell. I taped it up fast and dirty to take this pic. A tape and paper machine would help you make it a nicer job.)
Step 3: Get You Some of This Magic Ish.
Step 4: Spray and Pray
This sprayed on Bedliner stuff has a HUGE spray tip on it and it sends particles EVERYWHERE. It's got a 12/12 tip or something 12 inch circle at about a foot away. (It's a special magically engineered tip that is metric too- 12 cm away it's 12 cm across) ;)
Start spraying. I needed two coats on many places to get full coverage and I needed 2 cans to do the inside surfaces of my Wildernest (including the bed area)
Now the inside of your truck camper looks like an awesome love shack!