how do you get this stuff out of the inside of the door? some of it has fallen off in big chunks but whats left is stuck on like white on rice. im afraid to sand blast, i though about a putty knife but afraid of damaging the exterior door skin. i plan on using por 15 to treat the inside and i dont think it will stick to the undercoating. what do i need to do? Thanks Dave

I removed alot of undercoating on my car by heating it with a propane torch, then scraping it off with a putty knife. Instead of a torch, you could rent a heat gun. It softens the coating and it relatively easily scrapes off.
That coating is likely a sound deadner, not rust undercoating. Once removed, you may want to glue a new sound deadner on the skin. They are sold in sheets. It is a tar like square sheet that sticks like fly paper. Search sound deadner, sound dampner on this site for more info.

im afaid the heat will distort the exterior door skin renedering the door usless.

Here's one answer, although it is a messy one: oven cleaner. Yep, cheapest oven cleaner you can find, it'll come off like butter. How do I know? I spent a lovely week on my back under my '68. What a mess, and be sure you've got good ventilation!!!

Dave,
Using a heat gun will soften it up but you just don't want to get it too hot or use a putty knife that makes gouges in the skin.
I saw in another post about pin holes in your doors and if they are the same doors with this undercoat or sound deadener you may think about having them dipped. I found a little shop locally and after I got my parts back they were nice and rust free inside and out. It also showed me where there were rust through spots.
My doors and front inner wheelhouses had a real thick application of sound deadener in them from the factory. The factory didn't spray the whole inside of the door and front fenderwells but just put it on in like a wide strip. After I got them back from being dipped I was concerned as the stuff was still on the parts but after I got them home I was able to pop the material off using a dulled down putty knife and no heat. I think the dipping solution did something to it and it wasn't bad at all to get off. The metal that was covered up was still nice and rust free so the parts did not have to go back and be redipped.
The nice thing about dipping was that the solution was able to get into all of the nooks and crannies unlike sandblasting which is more line of sight.
When I had the doors all prepped I literally poured in the POR-15 products and sloshed it around making sure all area's were covered. I then let the excess drain out and whatever got on the outside where the final exterior paint would be it was sanded off after it had dried.
Jim/Saint Louis

Eastwood makes a product called "UNDER GONE" specifically made for your purpose.