I agree with a lot that is said above. POR stands for Paint Over Rust correct? When you use a product like that it specifically states NOT to clean the area to bare steel with media or any other abrasive. Personally that isn't a proper method of rust removal or corrosion protection. I have also used POR-15 and personally thought it was the worst product I have ever used. I wasn't using it on a rusted area just coating the back side of some fenders. It brushed like an oil based paint that left massive streaks and was VERY hard to cover, then no matter what type of storage method I tried, the cans got rock hard and 50% of the product was thrown in the garbage.
In my personal opinion these products would be good for the bottom side of a car, NOT rusted, for a nice finish if undercoating isn't wanted. I used the Eastwood stuff on the bottom of my car and it turned out nice. I would never use POR-15 again.
Here is an example of poor prep. When I got my dart someone had used POR-15 over bare steel, it was peeling on every inch of what they applied it to, and after hours of sandblasting to remove it, I applied the proper coatings to make it last. You can't just slap something over rust and think its going to "lock" it out. It needs to be repaired or replaced.
