Don't clean it.

I'll make it look old.. Spray it with cement dust and leave it. It will look like a shed in 5 minutes and look like that for ever.

An alternative is to use an angle grinder on some old steel piping and angle teh sparks all over the car. Will give the paint a nice pitted rust effect..

This is not a wind up when i posted it i did not think of it being taken like that but of course you are right to think it. IT is though a genuine request. I wish to leave it parked but not draw attention to it. I dont want it damaged but just to look old. The best option seems to be just get it as dirty as possible.