We took my undercoating off with a hot air gun and a scraper, helps to have a 17 year old college kid who wants to earn a few quid too.
I tried a couple of different methods, blow torch, stripper, scraper, and the hot air gun seemed to work best. No sandblaster will really do much against undercoating of any kind, it just bounces off, I guess if your man had a very big industrial model and just stayed in one area hard then it may do it but it would also deform the panel to hell. My guy had a very large model mounted on a 14 tonne truck with a hose I could have crawled in, and he said his wouldn't do much against undercoating.
We stripped all the undercoating we could get to by hand and then paint stripped the insides of the fenders, ie the back of any exterior panel as I didn't want the sandblaster to go near those, I only had him do the inside /outside of the floor pan, chassis rails, engine bay and boot, but not the back of any exterior panels. I also had him do the windshield frame and then turn the pressure right down and do a couple of spots outside the car where there were a few surface rust spots.
I had also already stripped the paint off the dash but I hadn't treated it with kurust as there were a couple of areas that I couldn't get with stripper so I had him very gently go over the dash too.
It didn't take long to blow the sand out, there will be a bit leach out as body work is done so by the end of it 99% of it will be gone. It is the method I used on the Mustang and it worked well for me on that.
I thought hard about dipping but read a mixture of reports, some good, some bad. I was concerned about them denting the shell moving it around their yard and during the dipping process, and concerned about some acid being left in box sections, may be unfounded but it worried me. It was also a pain for me to transport the shell there and back. I was quoted around £12-1400 to dip the shell and aluminium panels, I guess if it was going to need to be dipped again after the repairs then e-coated maybe it would have been 3 times that, in which case my way has been a bit cheaper.
I think if I were doing it again, I would still do it this way, the dipping is less labour intensive for me personally but I think this is a bit kinder to the car.
I would definitely remove the undercoating again too, I was reluctant at the start and it didn't uncover much more but it did uncover a bit so in my mind it was worthwhile. I know at the end of the day this car will be absolutely as good as I can make it.
The rotisserie came from autotwirler.us Ours is the pro model because we will use it a lot, and have been. It has become so useful to us as a tool that If I had a bit more room to store and use them, and could justify the expense, I would probably get another. My advice would be get or build one, it makes the job so much more pleasant when you aren't laying on your back. The autotwirler stuff is very well made and they are easy to deal with, it may not seem cheap on the face of it but when you figure the cost of the steel etc and then how long it would take to make to the same standards the price is ok.