I have these computer cases that come to me from the shop that fabricates them. They are sheet metal and come with an oil coating to prevent them from rusting. I have tried severals ways to clean them with a 30% failure rate (contamination). I have degreased them, power wash, oven bake for 1 hour at 500, the part temp after 1 hour is over 460, then sandblast. I have had them chemically strip, better results but still have contamination issues. What is the best way to remove this stuff so I can elimate the costly re-do’s? Thank for your help.