What an utterly messy job – undersealing the landy. So good prep was needed, first a plastic sheet on the ground (an old dust sheet) and reverse the landy over it. This keeps it in place and helps to cover off any over spray of the gunk. Next step was to cover everything that I didn't want coated – exhaust, wheels, brakes etc. I did this with a whole bunch of old packaging paper and a bit of paper tape to keep it in place. The underseal can go anywhere, so make sure you have a good wrap of paper with no gaps.
For the underbody seal we used some stuff from Machine Mart – Tetraschutz. It's only 5 quid or so for a litre and you can use a standard compressor and underseal gun for it. With only the rear boot section to do I used a couple of litres and put two coats on the boot area and a single coat on anything else I could see. The only (and quite a big) down side is that I didn't have the time or kit to jack the landy up and take the fuel tank off. So there is a whole area above the fuel tank that has not been covered. This should not be too much of a problem though as most of that area I have coated with zinc primer and a layer or two of hammerite black. I also took the opportunity to spray some in the chassis and wheel arches and well just about anywhere that looked like the original underseal was starting to crack or fall off.
I've got to say though that the Tetraschutz went on very well and has clung to just about everything it was aimed at. It took a day or so to dry into what feels (and looks) like fresh bitumen. Only time will tell how well it stays on there and how good it is at repelling dirt and water.
One note though this stuff sticks – it seems to especially like any exposed skin and it mostly likes to clump together a group of hairs for fun. So wrap up well, then wrap up again over the top of it