I want to por-15 the undercarriage of my 76 but I am not sure how much I will need and if I need to buy the prep stuff ?

there are many fans of por-15. I'm not one of them. the car i'm resto'ing now has many por'd parts and i removed it because of separation and water getting underneath and rusting the metal. It is a paint and requires the same level of prep to get it to stick and stay.

Eastwood IMO has a better product with so called proven test results between the two. Check their website. Eastwood.com and do the search on POR. An no I don't work there

Two votes no! I am not a fan either, A good epoxy paint or bed liner is a better choice.

Ted

Thanks for the advice ! I am trying to protect it from rust the best that I can and give it a clean look too

I vote no for Por15 too.

so any suggestions Brady ?

If you were to strip the bottom, I would use Master Series for a sealer then a quality bed liner like Ted suggested. Epoxy paint is great too. I used it on my car.
Check everywhere to see if all undercoating is attached. I found in several places where it was loose and easily pulled off.
That was one reason I stripped the bottom. I also found rust in between the undercoating and body that was hard to remove. This is a hard task to do just by jacking the car up. It took me a week just to clean mine on a Rotisserie.
Eastwood makes a material that is sprayed on the undercoating to easily remove it.