Is Rustoleum risky? No worse than any other applied liquid product, prep is everything. Exactly the reason some have problems with powdercoating.
Do you need an impervious hard coat on the frame that will last longer than other parts of the car? If so, then the expensive paints do the job, too, for more money.
The "trick" with Rustoleum is prep and sanding between coats, to get a smooth flat finish. It was originally dreamed up in the '90s as an alternative to $8,000 paint jobs requiring $1,000 worth of equipment to spray on liquid paint - with a 60% loss of the paint as it drifted into the air. With paint running $400 a gallon for red in the cheaper grades, you are spraying over half of it away with no application.
So, of course, the paint and body industry has a huge hate on over roll-on Rustoleum. They shouldn't - it still requires competent prep, bodywork, and sanding, and the results of poor work still telegraph the lack of professionalism. What micro thin layer of paint on top is really biased to the application technique more than being technically superior. It's mostly clear over just enough color these days anyway - another further complication.
I've used it, it works, so have hundreds of others, Hot Rod Magazine did an article on it which I feel is a fair review:
http://www.hotrod.com/techarticles/b...get_paint_job/
Painting the frame means prep, and there's a lot of it. It has to be degreased, cleaned, the welds dressed as much as needed, then some kind of primer applied to help the color coat stick. That's no different than any other application, and that's two details the average DIY will make mistakes doing. It's labor intensive - which is most of the expense in a custom paint job.
Paints that apply directly to bare steel have the primer adhesive ingredients already blended in, or use materials with that ability. I use rust prep spray on coatings, the ones that turn rust into a black inert layer. I've done bike frames, pool ladders, and the roof on my Cherokee, it works. The ladder was the most abusive situation, it soaked in a chlorinated pool six months of the year, and it only rusted where the tubing scraped the upper chine. Basically, it was scratched off - and any paint system will fail in that application.
Once the rust converter is applied, you might still see where there was some rust, which would be minimal after all the cleaning and dressing with a 3M grit pad. Nonetheless, it tackles that part of the job that would otherwise never be detected by blowing an opaque primer on the tubes - where other paints will eventually fail.
Mix the Rustoleum of your color choice - there's a choice, you don't have to live with black - 50/50 with mineral spirits. Fill a roller tray, with tubing, use a smooth foam roller about 2 inches wide sold at Big Box. Apply. It's the consistency of milk, go easy. If you choose to apply Rustoleum direct, that's ok, there would be no sanding or additional coats.
The 50/50 mix dries extremely fast, less than a day and it's rock hard. Straight Rustoleum may take up to a month - the solvent has to vaporize out of the drying film and it takes longer. Heat lamps won't hurt at all, a summer day with temps in the 90's helps a lot.
The result will be having a repairable coat of paint that is durable on the frame for less than $100. You be the judge.