Looking good.
Whats the fibreglass filler like to fill and sand?
Filled my backbox with Builders Bog - Crap  -Applied it on Monday & attempted to rub it back tonight & its still soft so I scraped it all off.
Used Gyprock top coat on the front of cab and found it good, dries quick & easy to sand.

great stuff as long as you sand within 24 hours, after that its time to get the angle grinder out.
but if you dont put enough hardener it wont cure properly either,
i have a disk sander and two orbital sanders (normal one and a mouse type) i get it close to level with the orbitals and finish off by hand then fill the last little imperfections with the standard car bog.