Here in Finland we have green scotchbrite pads and those seem to work pretty good. You still need to press the pad against the paint pretty hard while rubbing so that it really dulls the surface.
Word of warning about por15, don't ever use it on bare metal that has not been treated with acid. Seems like the acid prepping mentioned in the instructions is really needed. I found peeling por on several places on my doors last weekend and I was pretty bummed about it. There was rust under the peeling por which was strange because there was no rust issues on the door before painting.
The surfaces were roughed with sanding paper (150 grit if I recall right) and thorougly cleaned with solvent before painting with por.
On the other hand, the por seems to hold excellent on surface rusted surfaces without any major prep so only the clean/new metal seems to be a problem.