what primer to use under urethane enamel

Rod

good prices good service

My personal preference for a bare metal primer is a two-part automotive epoxy. I use PPG's OMNI MP170. It should be compatible with any topcoat from any manufacturer. Such epoxies are not available in a rattle can because they have a short pot life after the components are mixed together. They are iso-free however. For small parts that don't justify using you spray gun, you might want to consider the very inexpensive Preval Sprayer. The power heads are only a few dollars. I wouldn't want to paint a tractor hood with one but they do work with the epoxy I use for small parts. I've used one w/o the glass container - using a throwaway 3oz paper drinking cup instead. It takes two hands that way but works well with essentially no cleanup. The thing you should be aware of with the automotive epoxy is that if you go past the recoat window (3 days for the MP170 - different for others), you should scuff the part with a Scotch-Brite hand pad and reshoot a coat of epoxy prior to topcoating. It really is not as much trouble as it sounds. That scenario will provide you with the very best protection of your cleaned parts until such time that you can complete the painting, even if you leave them out in the rain in the interim. Or even spill gasoline on them.

Another option for you is to use a product like Picklex 20 to protect the metal from rusting until such time that you can prime and paint. I've used that method very successfully when it was too cold to prime. It's fairly pricey stuff but goes a long way and is worth it in my opinion.

The least expensive commercial fresh air breathing systems that I am aware of are the Breathe-Cool system from Turbine Products (also available on Ebay) and the Hobby-Air system from Axis Products. Both are available in different configurations from on-line suppliers. I have never personally used either of them. There are others that post here that have used one or the other so hopefully you will get some input from actual users.

Rod