Outgassing a valve cover in prep for powdercoat isn't necessary, IMO.
I just have a local place do mine. An old DSM buddy is the guy that does the work. Quality for cheap. He's done valve covers, brackets, the crossmembers under my car, oil pan, and even my summer wheels for my Audi. No parts were outgassed, no issues. The coatings have held up for years.