My $.02...
I use both, RUST BULLET, and POR-15.
W/ the 15, you MUST put on an overcoat, if it is exposed to UV light. I talked to the tech at POR, about using it on new metal. The answer was no.
Rust Bullet seems to be a popular coating, and acts much like POR.
As Bodhi mentioned above, a rust killer can be used, and then a topcoat used.
To kill rust, and prevent corrosion on bare panels, such as new, or blasted items, I use A MUST FOR RUST, from Home Depot. [There are several products under that name. BE SURE to read the labels.] About $8.00/btl.. will do an entire car, if applied right!
I put it on w/ the sprayer that comes in the bottle. Do a THIN mist, then scrub it in w/ a Scotchbrite pad, and let it dry. DO NOT puddle it, as the puddles will not dry, and will mess up sandpaper.
Once I get ready to finish, I wipe w/ prepsol,[wax/grease remover] run the D/A w/ 180 on it, over the area, wipe w/ prepsol, blow it clean, and paint it.
If you choose to, you can knock off all the loose stuff, blow it clean, and put on the POR, ??, etc, and then put the carpet down.
If the work is in the trunk area, I'd seal it w/ the chosen product, such as POR, then shoot the Zolotone,[spatter paint] over that. ZOLOTONE is water based, and MUST be applied over a sealed surface.. That is unless you want to see the rust start creeping thru the finish...
A neat "trik" is to mix about 25% clear in the last coat of ZOLOTONE. It makes it a MUCH more durable finish, with just a hint of gloss.