Bud:
I have used Rust –bullet and Por-15. While they’re both good and they both have limitations.
Rust Bullet comes in silver only and you need two coats.
Por-15 Gloss Black will break down from UV exposure and therefore needs a top coat- Not easy as this is so slippery you have to sand it or use an etching primer first.
I prefer the Por-15 gloss black. You will note that in none of the tests do they ever use the gloss black, instead they pick Por-15 silver, which I think is vastly inferior to the black.
There are rusty places where you can’t sand or remove the rust. Places such as inside heater channels, behind fender wells, inside rocker panels. This is where I have found Por-15 to be superior. I have patches holes using fiberglass and Por-15 instead of resin.
Because I can paint right over rust, lay the glass mat down and paint the fiberglass. This gives me a rock hard surface. However this is not my idea as I stole it from Hot Rod Magazine where they repaired a rusted out floor on a Chevy and repaired a rusted out rear window channels.
While I never did any scientific studies, my neighbor gave me an old fold-up trailer.
It was an eye sore because the faded red paint was 95 % rusted. Every time I used the Por-15, I would wipe the brush on that rusty trailer-never sanded or wire brushed the loose rust. One of the unique feature of por-15 is you can push the paint. It seems that a little goes a long way. Anyway after some three years of whipping my brushes on that rusty old trailer, it was all black and it looked good, good enough that I was offered $125 and I sold it.
One note: Gambardella Speed shop sells Por-15. No waiting for the mail if you need it fast- Same price as on line and no shipping charges.
Gambardella’s donated a $75 Assortment of Mothers products to our Christmas party for door prizes last year. I hope if you need Por-15 you will buy it from them. Also they can get you any of Pertronix electronic conversions for your VW and the correct coil in about 2 days.
jt