no you don't need the guard, use some good glasses or a face shield ... angle grinders and flap discs don't work well really flat, although the flap can gouge easily at too much angle, and they're most useful
for initial stripping, not finish work .... you need a DA sander after the flap, using progressively finer paper to at least 300something before
initial primer, then using that primer as a
'guide coat', finer sanding before
more primer and paint
if you don't have a DA [or air] use a quarter sheet palm sander - lots of sanding, finer and finer, is the key to getting rid of the work marks
Bill
