You do have to rough up smooth metal before POR-15 will adhere, and used on flexible surfaces such as leaf springs it will crack and flake. If not exposed directly to sun, it will be fine. If it is, a clear top coat is needed. It will just chalk up a bit, but a bit of wax will bring it back. Just remove loose rust, wipe down and POR it.
I still have not seen rust bubble up on properly coated surfaces, but I've only been using it for 30 years.
