get the sprayable kind from home depot or walmart... cake the bitch on and wipe off , repeat as necessary , don't try to hose it off untill you think it's like 100% clean otherwise it'll stop the chemical reaction that strips the paint.
sand and paint but make sure that you clean the surface before painting no oils nothing and i would sand with 400 grit and work your way up to 1000 or finer grit ( wet sand) to get the best finish