Mark

You need to be carefull on what surfaces you are cleaning with certain types of chemicals as far as pressure washers are concerned if you are using a cold machine its better to use a honda petrol or a more expensive diesel machine if you are going to do a lot of work, i have  13HP HONDA PETROL at 13lts per minute and about 150 bar this machine has never failed to do the job and is vary reliable you can get some cheapies from china but they don,t last.
we use different kinds of grafitti chemicals and also do a indoor/ underground used on plastics/perspecs etc very safe just spray on and wipe off with a mild scourer and water. if you need anymore info giz a shout