Just to relight the fire on this one guys it stops after the sanding of the filler. what are the next steps afterwards? Is there any fibreglass of filler that needs to go on the faceplate or is it straight to priming?
Any exposed paper/cardboard you'll want to petrify, yeah. Otherwise you'll be sanding raw cardboard, which is about the least sandable material in the world. Pretty much anything that soaks some into the cardboard and hardens will work, but yeah, fiberglass resin (without the matting) is a popular option.
So yeah, petrify any exposed cardboard, then get it as smooth as you can via sanding and filler, then start priming. The initial priming layer will mostly just be another filler coat layer, for any tiny little scatches and such, so thick filler/sandable primers work best (I'm a big fan of Rustoleum '2 in 1 Filler and Sandable Primer').
Layering, sanding, then layering again is gonna be a long process that takes up the majority of the time of any scratch build, but putting the elbow grease into it makes all the difference between a good clean build and a lumpy one.