If you've sanded the car down to bare metal.. you don't really need to use metal prep.
It is a good product, but has it's quirks.
for about $15.00 you can buy a can of aeorosol bare metal etching primer.
LIGHTLY apply a "dusting coat" over the bare spots only.
followed up with Epoxy primer, if the car's sitting for a while, or you need a barrier coat.
if it's a straight panel, and only needs prep work for painting.
Then use Urethane primer directly over the etch primer, block, reprime, and you're ready for paint.
Whatever you do though.
try to avoid using either the metal prep, or etch primer around body filler.
"Bye" Bye" Riceboy
(Ex-wife fore sale... take over payments)