Solo my friend you've been given some bum information. ZR is as good as any surface it is shot onto.
If the rust is scaley loose, ZR will fall of "with the scale" and you won't get it off the scale.
It is the perfect product to use where you have tight rust and shiney or not so shiney metal on the same surface.
ZR is a sealer ... POR is a converter which chemically converts rust to something akin to porcelain ... it does not work on shiney metal ... nothing to convert.
Get Bruce Palmer to send you a brochure - it lays it out nicely.
