Oven Cleaner? Fuck sake i bet the paintwork was in some state. Try to get some Autosmart Tardis, spray on leave for a few minutes then the Tar just wipes away with no effort. Don't use a machine with T-Cut unless you know what you are doing. Don't even touch the car with a Rotary or DA unless you know what your doing first. You could try something like Megs Scratch X to sort scratched bits. If you really want to use a machine get a scrap panel from a breakers or something and practice on that mate before you do the car.