I'm going out on a limb here, but I thought por-15 was a coating for body repair, and it sounds like you are doing rust proofing. I would reccomend taking the car to a rust proofing shop. It should come in well under 200 dollars, with a 45 dollar 'touch-up' charge every 2 years or so to keep it in the best condition. Just my .02. For an good set of directions on por-15, check out www.vclassics.com
and check out the tech section. The site is volvo based, but the same idea applies.