Hi Fireman,
The £500ish bloke himself here
Remember you get what you pay for!
If it is not too bad you might have a bash at doing it yourself. Remember this rough rule of thumb. If I do it:
on light to medium rust the waxoyl will kill the rust and stop any further rust for about 5 years. On very heavy rust it will only slow it down for a year or two . If it is very rusty, there is only one place that can do it properly I'm afraid. If it is not too rusty and you decide to do it yourself here are some tips:
1. Pick a warm day. The waxoyl will flow much better so you will not have to dilute it too much. I would advise against doing it on a very hot day because the waxoyl will be very thin and you will not get a good coat.
2. Use a proper rustproofing gun (they are not too expensive nowadays - do a web search) and hire or borrow a small compressor for the day.
3. Heat the waxoyl gently before use if its still thick. Do this by putting it in a bucket of hot water. For safety you should really take off the cap if the water is boiling but if the water in your bucket is not boiling you might take the risk and leave it on. Don't tell Health and Safety I said that though!
DO NOT UNDER ANY CIRCUMSTANCES try heating the can in or on anything other than hot water - because you would be putting your life in danger!
Once the waxoyl is nice and warm, give the can a damn good shake. You want to do this for a few minutes until you are knackered.
4. There are businesses which specialize in pressure washing the underneath of lorries and trailers. You might find someone locally to do this for you.