How about washing it with a high pressure hot water washer. The unit I own heats the water to 180 degrees. I then douse it with Dawn dish soap. Let it set for five minutes or so and then hit it agin with the hot water high pressure. Remember high pressure will/can remove anykind of paint / finish. Just be carful. As far as the soap ( dawn ) goes I have not ever had anykind of trouble using the soap and then painting. The paint I have been spraying was always ememal.
