I use the Cermark LMM-6000 spray. So far it has worked great on non-magnetic stainless and aluminum. It always leaves a very dark permanent black mark.
The only time I was less than happy was when I tried to engrave some really cheap stainless steel whistles. I later discovered the whistles were low nickel magnetic steel. It did mark them but the color was not nearly as dark. So I learned that not all stainless is the same. But for high quality / high nickel content steel it works great.