Can I spot fix areas on the underside of my hood with POR15 then after it cures ruff up and primer and paint the whole underside???

Yes. Por makes a product called Tie Coat Primer. It lets you do just the thing you're wanting to do.

i see a lot of reference to por 15 is there any one place that is more reasonable price wise to buy por 15 or is it the same eveywhere?

It's about the same everywhere. It's expensive stuff, yes, but a small amount goes a long way. People tend to buy more than they will use.

You might try ordering direct.
www.por15.com (http://www.por15.com)

Maybe it'll be a little less than a local supplier. It's worth a look.

Buy it direct, it's your best price. They will also send you their catalog, which has some interesting stuff in it. I've used their products many times--good stuff.