With EVAPO-RUST, do parts have to be completely clean before immersion?

No. EVAPO-RUST^TM has detergents which penetrate oils and dirt. However, surface contaminants do slow the process. The soils must be penetrated before EVAPO-RUST^TM can come in contact with the rust underneath. Soil residue will also shorten the life of EVAPO-RUST^TM. Heavy greases and cosmoline should be removed prior to de-rusting.