I've used POR-15 for both my under-carriage and suspension parts, with
very good results.
A few thoughts:
 Purchase several of those inexpensive disposable foam brushes, for any
bristle brush will be impossible to clean and useless afterwards. Also,
I've found POR-15 applies more evenly using foam, resulting with fewer
drips and ripples.
 Make sure you purchase only what you need. The shelf life is only six
months, and whatever unused portion will most likely harden in the
container unless you transfer it to a new smaller sized container. I
transferred the balance to "baby food jars," filled to the top, and sealed
tight. Was liquid later, be still only good for 6 months.
 Wear a hat, goggles and gloves. The stuff will come off skin with
significant scrubbing and perspiration, but not from clothes! Some of that
Orange cleaner with a scrub brush remove it from skin, but not something
one should do regularly.
 As for leaving surface rust, I found using a wired brush to remove any
'excessive' rust good practice. I haven't experimented with their
'metal-prep' but find contradicting info on their website/catalog. They
say to use it for better adhesion, but then also claim it is not necessary
- I assume that the paint requires a rough surface to bond to full
strength. Although, I've coated my original lug wrench with it which was
smooth as glass with good results.
 There are some Dealers of POR-15 which discount... look around for
better pricing (about 10%)
 Stay away from some of their other products, namely Boss Gloss and
Artisan Metal Polish. Performance is nominal. While they offer 100%
satisfaction, the return post would off-set the refund, and there are
better products for less money available at your local auto/hardware