I will be applying POR15 to may frame here soon. Whole thing is covered in surface rust. My question is do I need to use metal prep since it’s rusted or is that only for clean metal?

 

While you can paint right over the surface rust with POR-15, the Metal Prep (which is just diluted phosphoric acid, so you can use that instead of the POR-15 brand) lightly etches the metal (with or without surface rust) so it will help the POR-15 adhere better. Perhaps more importantly it also stops and seals the oxidation that's happening to cause the surface rust in the first place, leaving behind a zinc-phosphate coating.

Be sure to let the Metal Prep/phosphoric acid do it's thing but wipe off any puddles (multiple light applications are better than a full bath) and let it dry completely before you use the POR-15.

Also, be sure to top coat the POR-15 with multiple light coats of a Chassis Coat/Top Coat or similar to protect it from UV rays as well as road wear and tear.