Hello Jerry,
Regular primers will not stop moisture from getting to the metal. They are designed only as a basecoat. It will only take a week or two before rust shows up on exposed primer. Most primers condition the metal and create a good bond for finish paint. Think of them as a "sponge" very porous, to catch and hold the finish paint. The finish pain seals off the system. Aerosol paints are thinned excessively so they can spray out of a can, so they provide the least coverage and protection.
Some more advanced primers (epoxy primers, primer/sealer and commercial two stage primers) will do a better job as a sealer. But non of these are intended to be left exposed to the weather for a long period of time.
Here is one easy solution for the amatuer, you can buy some Rustoleum "Rusty Metal Primer" it is a very good bare metal primer to go over bare metal. It is easily available, inexpensive, brush on, or spay can, (brush is best). I think a quart can runs around seven or eight dollars at almost all department stores or hardware stores. It has fish oils and a lot of other conditioners for preventing rust. It is great for floor pans, frame channels, underbody etc, Use a semi-gloss or flat black rustoleum to seal it off. The non-gloss finish will accept your finish paint color sometime in the future. You will have plenty of protection until then. However it will need a day or two to completely dry.
Many more exotic and more expensive products are available from companies like "The Eastwood Company" check it on the web.
Joe Alexander
Blacklick, Ohio
