where as por 15 is very good stuff, it only sticks to surfaces with a "bite" like rust...or really roughed up surfaces....the (gummy)seam sealer spots, and the smooth paint spots will more than likely reject it..(it'll peel)...a simple killrust or the above mentioned products would probably do better for ya.
I used rust bullet on my floorboards. Scuffed them up a little, wiped them down, and painted it on. It took a couple of days to dry fully, maybe almost a week, but it is hard as a rock now. Here is a pic...
It's covered with Hushmat now, and will probably never see the light of day ever again!!