Stainless, I use Simple Green to clean my bike, boots, helmet, chest protector, air filters, and everything around the house. The stuff is great. I've been cleaning my filters with it for over a year with absolutely no problems. I'm guessing you had some type of a chemical reaction from mixing the No Toil with a degreaser. Well next time I go to the store I'll have to grab some simple green. I like to use degreser on my bike at the pressure washer. Is there anything I shouldn't use it on?