What would be better Por15 or Zerorust? Will they work over primer? I'm restoring a '70 Nova with light rust, have already cleaned up and primed front subframe. Would like to coat floor and firewall also.
I have not tried zero rust, but I can recomend POR-15. Por 15 is a great product, its a bit spendy compared to zero rust, but you get what you pay for. You can't apply POR-15 over primer, it has to be bare metal so the por 15 can bond with the metal and create a sheild over it. I recommend it fully as will many of these guys.
I went with zero rust for two reasons: no isocyanites in it and it's MUCH cheaper. It's easy to apply and a PITA to grind if you decide you need to weld where you've painted. IMHO, unless you're parking your car in a salt water swamp, both products should perform suitably.