We may be talking about the same thing but i used Evapo-rust that I bought from Harbor Freight. It used it to convert the surface rust on the bottoms of my doors. I put it in a spray bottle, sprayed it on, let it sit, and washed if off w/ soap and water. Worked well...took care of all the rust and allowed me to scuff and prime. It's pretty inexpensive...and environmentally friendly. Best of luck.