primer and gas
Not sure what you are asking. Are you thinking of flying in primer.
Primers are not necessarily good top coats. If you prime and fly, you will be contaminating it with grease, oils and fuels. You will need to rejuvenate the primed surface, at least requiring sanding off the outer surface and cleaning it well before painting. Depending on the level of contamination it may need to be stripped.
However I think a primed surface is better than bare fiberglass, which in the case of the cowl has lots of pin holes and can absorb some contaminate. As far as aluminum, if you are not going to paint before flying, I would not prime until ready to paint. Most primers are designed to be painted with in a very short time period after application. Some must be he shot with top coat in a few hours. The paint experts on the list can fill you in better.
George