I use a product called Durethane. It is a product sold by most Porter Paints stores. It is a DTM ( direct to metal ) product. Do not put it over bare aluminum, but it does well over bare steel. I probably buy 15 gallons a month at minimum, but have had projects that required many hundreds of gallons. The nice thing about it is that it cost about $85 a sprayable gallon and it is a 2 part product. Certainly on the industrial side of things, but has good UV qualities and can be sprayed, brushed, rolled, etc.
I also like the SEM Rust Shield. It is a lacquer based product and probably runs $140 a gallon. It is not catalyzed, but seems to hold up will on chassis and on under hood stuff. The nice thing about it is that you can paint a little and then put the unused paint back in the can....
I do a fair amount of restoration work with my business and I think that both have pros and cons. If it is a bare frame that I can put in the booth I would use the Durethane. If it is a frame that has been cleaned up with wire wheels and such and has not been blasted I would probably use the SEM stuff and not worry any more about it... My 46 in my signature below has the SEM stuff on it and it looks good for what it is!
Lots of opinions to choose from and you should find the one you are most comfortable with doing and don't second guess it!
Good luck with which ever direction you choose.
