You trying to protect it from rust or remove treat the rust? Never used POR-15, but read about it and the prep sounds like a pain.
I used this awesome called "ONE-STEP" you can get it from any auto paint store. For a medium sized bottle cost me $30. I used it on my CJ cause the frame was all surface rust (it has to have rust to work). Looks like pepto bismo, you put on a couple coats with a paint brush and it turns the rust into a hard black primer. Indestructible, hard to grind off, seals it good and you can not weld near yet as it messes with your mig gas. It made the frame look like it was fresh off the showroom floor!