I Jet washed everything, then used a grinder with a wire brush attachment to get rid of the harder to shift surface rust. I then gave these areas a coat of red oxide primer, then once dry, applied a coat of hammerite with added waxoyl. 10 months on and I've had no break through rust, and the MOT guy seemed pretty impressed.
This method requires a considerable amount of prep work and a lot of hours under the vehicle. My advice would be to work on a small area at a time, rather than become danted with preparing the whole of the underneath in one go. I'd also recommend a good pair of safety glasses and a respirator style mask, you'd be amazed at how much crap comes off from underneath!
After I did all that, I gave the cavities a good blasting with waxoyl, and job done! From what I can see of it, waxoyl should only be ever be used in box sections such as sills and chassis. I spoke to several chaps before doing mine, who told me that if you spray the underneath with waxoyl, the exposed waxoyl surfaces get bumped and scratched. The waxoyl will spread and recover these scratches, but it then seals in any water that the exposed metal surface may have come into contact with. This speeds up the rusting process, and your vehicle will rust a lot quicker!
No doubt people will disagree with this method, but it's just my view. Read up as much as you can before you get underneath, and do what you think is best
HTH,
Lew
