So with winter coming up I have the time to do the things I wanted to do to it that I skipped during this past summer.

My 86 is practically rust free. It does have some surface rust in the usual pit spots that go to hell in our cars.

The car does have what seems a rough undercoating in some areas. I will probably have to chip those areas off, but besides that it looks like its just plain old dirty.

How much will it take to completely coat the underbody and have some amount left over for other projects. I assume getting 1 gal would be too much? I want to purchase a Quart and see how much that covers, I can always get more.

I have heard people say if its practically rust free that just the coating itself will do and won't require their other steps.

I will be doing this very soon, as soon as this weekend since I plan to drop the rear end and tank. So probably will get the rear half done.

Please chime in those who have done this.

When I dropped my rear end and tank I found little or no rust back there. The rear impact bar was rusted and had to be replaced. I've been using this stuff on all my vehicles for the past year or so and have found that the whole system, meaning the marine clean, then metal prep, then por 15 works best. I would guess you'll need at least 2 quarts if you want to have some left over.....especially if you start doing things inside the car. I bought the 6-pack last time and I keep it in the fridge like they recommend. Just make sure you prep the rear portion of the car after you drop the rear.....I don't know how well it will stick to the coated metal back there.
AND BEAR DOWN!!!......nice win yesterday

Por15 should be used over rust to be most effective and isn't really designed to be a top coat....so they say. Check out Rust Bullet and Eastwood's Rust Encapsulator for comparisons.

I've had good results with this stuff. Plus, it's cheaper. http://www.magnetpaints.com/underbody.asp

I've also used this stuff for my entire underbody: http://www.masterseriescoatings.com/index/ag111/

I use POR15 on most of my interior and my floor pan and underneath the car about 90% coverage with 2 quarts.

POR 15 is for rusty areas, not for putting over paint, it wont stick well.
I sure wouldnt scrape off factory undercoating to put this on.
You would b better off using POR just on rusty areas THEN a spray on bedliner (under floorpan) and something like lizard skin (inside floor pan) they stick to paint

I used mine inside and outside mostly because I replaces some parts of the floor pan. After I welded in the new metal I got it wet and let it surface rust then I preped according to the directions and painted.