The first thing when I picked my car up was to have a look at the under body rust. After losing an Alfa 156 to rusty sills, I'm not getting caught again. There's little point in putting waxoil onto rust, as it will only hide it, not stop it.
To start with I jacked the rear end up and supported it with axel stands. Using a wire bursh I attacked any obvious bits of rust, the rear towing eye, rear cross member etc. Then once done I gave all visiable rust a coat of Krust, this is supposed to kill the rust and turn it back to metal. Once that had gone off (2-3 hours later) I gave everything I'd coated with Krust a spray with some Metal BBQ paint from B&Q. This is a long lasting, heat resistant direct to metal paint thats also has anti-rust abilities. I actually did two coats, one in black and then the second in silver to match the car.
Then I swapped round the front and repeated. Again the towing eyes, and parts of the sub-frame where the worst offenders.
The next step having treated the rust will be to waxoil it.