[HO455]

Por15 is a good product. The problem in my experience is that there is no simple prep for topcoating. I too have topcoated it when the Por15 is tacky or still has drag to it but not to a 100% success rate. If you miss that window then you have to hand scuff the Por15 to get the topcoat to adhere. If you provide tooth for the Por15 to grab on to it will stick. Either solid rust (not lose or flaky rust) or acid etched clean steel. Then getting the proper tooth in the Por15 for the topcoat is the key to getting a nice topcoat that lasts.
As far as hot rod builders not using it I would bet it is from an economic standpoint and not from a product failure issue. Using Por15 correctly is a labor intensive project that quite frankly isn't in the budget of many projects. Especially cars that only see the road on dry sunny days. Not to mention the fact that today's standards of street car builds are more than the show cars of 15 years ago. Economics mean that as a builder I will get a new frame from a aftermarket vendor and paint it with a good single stage paint. That will allow me to have a sharp looking product at a good price and it will also allow for easy touch ups that will follow as the owner uses the vehicle. Powder coating and products like Por15 cost me more money to apply and in many circumstances I can't charge more for that extra time. In addition they make invisible touch ups a pain even though I can charge for those.
Now all that being said I have used Por15 extensively on my Burban and my Javelin to good success. It works well on projects that are not being completely disassembled and restored, especially year round driven vehicles.