hey,
well for mine i sanded using 80 grit and then went up to 120, that gives you a much smoother surface. its best to sand using a high grit after the bondo process just before you want to give it the final finish as this can take away alot of the scratches a lower grit can make..
for example i bondoed my bicep piece then sanded using 80g then 120g, then primed to see any defects (like more scratches, dents stuff like that, if there are then just sand them away in the same method)
after that if you think its fine, just prime with a dark grey or black one coat, and then after that is your final colour. but the spray brand is up to you to decide as there are so many out there.
anyway hope that helps, thats the way im making my armour.
nat