Since you are not using real auto paint products, I suggest use a self etch primer like can be had at auto parts store, then come back over it with their brand primer/sealer, then try your rusteolum enamel. Probably will be compatable over the sanding primer/sealer.... You will never get the lasting results you really desire unless you get at lest a small compressor, and cheap HF spray gun, and invest in pro products from the auto paint suppliers. A few hundred $$. No that little compressor is not adequate but yo can get stuff done with it.
There are reasons body shops use their stuff. My 2 cents worth.
