Leaving any primered car outside and exposed to the rain and the sun is a bad idea. Primers lack UV stabilizers and so will degrade when exposed to the sun. If you gotta leave it out, you at least should tarp it to keep the sun off, bu tyou also need to keep it dry from dew/condensation...

In general, I think the literature favors epoxy's ability to seal out moisture and oxygen over that of etching primers. Note as we've discussed above, that if you let the epoxy dwell more than a week, you'll need to scotchbrite it prior to applying the next coat.

Astrobuf