Just finished scraping away the old undercoating. I've scuffed up the old primer and scraperd down a few small rust patches. I'm undecided whether to use a regular automotive primer or POR 15 under the asphalt coating. Will the POR 15, being so slick and strong, allow the undercoating to chip and peel away. Will I be alright if I scuff the POR 15, or is the POR a bad idea for this application.

I don't have first hand experience with this but I understand that POR wants you to shoot your primer over tacky POR. If you do that, you should be able to shoot your undercoating over the primer.

You are right though. If you paint over dry POR it will chip off. If the POR does dry, they recommend using a mist spray of primer over the POR before putting any serious paint on it.

One thing about it though, the POR won't come off.

And it will be tough as all get out to scuff the POR when it is dry.

K

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64 2 Door HT 6-230 and a 'Glide (In transit from Cali)
Formerly 71 Elco

Chevello is right. You must get the primer on while the POR is tacky.