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.
Hmm,
Generally builders bog should setup within a couple of hours for sanding. Maybe not the correct mixture of hardner went into it?
