Paul, I've used a lot of POR in the past, leaving the rust in place and applying POR gives the effect of the piece looking like it has leprocy. ZR may chip when struck with a hammer. The price difference and the toxins involved when you compare POR15 with Zero Rust is a no brainer. Zero Rust wins "every" time.
I've just started using zero rust and I'm blown away at how easy it is to use and how cheap it is.
I've used 3/4 of a gallon and so far I've done two coats on my cab inside and out, top and bottom plus my frame and all that hangs from it.
This stuff covers like nothing I could imagine. And you barely need a fan to protect you from the ingredients.
I find it hard to believe that even whacking it with a hammer would have a negative effect.
Just my opinion and my opinion only.
