Painting - Painting Heating Vents

Need advice on repainting heater vent outlets and housings. Our house was built in the mid 40's. It has floor level vents that are about 1 foot square. Have tried in the past to repaint, taking vents down to bare metal, priming with Rustoleum or similar product and repainting. Within two or three days, small rust spots bleed through and eventually dominate the appearance. Any ideas on what to do differently? (Other than buy new vents!)

What I would do if this was me haveing this problem is strip off the paint and start over, I use bix stripper, it comes with a spray bottle that would help a lot in stripping vents, after it is clean lightly sand with a 220 grit and re-prime with a oil based BMP (bare metal primer) from your local pro paint store, then I would use a steel structural enamel or an industrial enamel. Sherwin Williams carries a spray bottle (and the primer and enamels) that you put your own paint in, this would be perfect for the primer and paint, be sure to read the instructions and thin the paint properly to use this spray bottle, allow to dry thoroughly before replacing the vents.

This is an overkill I know, but you used a rust inhibitive primer with the rustoleum, this would be my next attempt. BTW, what type of paint did you use?

Hope this helps,