I used the POR15 for the interior this spring, along with foil-backed insulation and new carpet. I was delighted with the way the floor came out and am considering stripping the undercoating that the PO used in the boot and using the POR15 there, too.
Preparation is the key to the success of this product...first use the wire brush to get rid of loose rust and old paint...then POR's "Marine CLean" which is a great cleaner, but requires rinsing with water, as does the "Metal Ready" prep material that POR will sell you with the POR15 kits. Sounds like a hass;e, but with good weather, it is not really that difficult.
One word of advice...only open what you think you will use in one application...once the POR15 is opened, it doesn't last in the can! I did 2 coats of the entire interior (plus a couple of spills in the driveway) with a brush with one quart of the POR15. I had some left over that I used on parts of my rusted boat trailer without any prep work and am amazed at how solid it is.
POR15 can also be primed and painted with other products too...or so they claim...with proper preparation and planning. I am going to get another pint or two along with their primer and use my car color's paint in the boot...someday.
Bruce