Just to echo Mr T
I tended to start my major repairs with fibreglass matting, before switching to fibreglass body filler and then finally finishing off with ‘normal’ lightweight body filler.
A top tip that Viatron/Mac
gave me was to start the first round of rough sanding as soon as the filler was hard enough to take it, but before the filler had set completely rock solid.
Whilst this wasn’t always possible, it did reduce both the time and effort required for sanding.
Good luck, Paul.
I know it isn’t 100% complete yet, but your bodyshell extension now looks like it has always been there.