Worked for a seafood company once. Took a small coffee can full of the acid they clean food trailers with and mixed it with a small coffee can of Gyro soap that is made to clean the bilge on ships ( the soap is designed for salt water so you need the acid to change the water) poured it into a 5 gal bucket of water. Put the siphon hose off the steam cleaner in the bucket and turned everything on. You could watch it take the diesel smoke off the sides of trailers. They would look almost new by the time you were finished. Just keep the wind blowing from behind you and wear a face sheid. Burns a little if it gets on your face.