Hi Ollie,
My 90 is going to get a good slosh of waxoil, probably in the spring. I have had quite a few tips and tricks given to me in the last couple of weeks.
First off- no reason not to do it yourself, it sounds as easy as it gets. If you can get some ramps or something to improve access it'll help.
Preferably jet wash and then wire brush off everything, the better job you do here the better and easier the rest will be. Then youre right, apply a rust eater type product which will chemically stop rust and turn it black. Then (and this is where youll hear of loads of different techniques) the wax oil.
One of my workshop technicians suggests using old engine oil, deisel and waxoil as its cheaper alternative, and a friend suggests watering it down with (i think petrol, but dont quote me on it) and then using a few disposable garden sprayers to coat the underside of the car. In this way, although the nossels will block with wax, if you have a few at the ready, you just chuck one and fill up another. Using a sprayer of some kind is advisable as it will get into all the knooks and crannies that water and grime will get into.
Another friend used an airline to spray his. Not sure about blocking with the compressor- it might have been powerful enough to blast it all through.
And all said just cover everything, chasis steering you name it!
Have fun! Rich and I are going to do his rangie and my 90 together for a laugh.....if you can call it that!
Ross.