Nothing you can buy in a rattle can is suitable for use under modern auto paints.

If you have an air compressor, you can spray epoxy primer. Dupont, PPG and SPI offer epoxy primers.

Harbor Freight and others have low cost paint spray guns. For a small compressor, the HVLP type of guns won't work because they need lots of air. There are pressure feed conventional guns that work with low cfm.

Properly sanded epoxy is a satisfactory base coat for nearly every modern finish. It's fairly easy to spray.