I actually got spots on DGS's dress shirt recently while using Best Press. The shirt is a very light blue, 100% cotton, but says that that it is "wrinkle free" so it must have been treated with something???. My iron is new and has a stainless steel (I think) sole plate, any way is not a coated surface. The spots were not discolored or dark but looked more like water spots or a grease spot would look. I did notice that the spray kind of beaded up on the fabric, but didn't wait for it to dry. Luckily, I did this early in the day and was able to re-wash the shirt and the spots came out. He was on his way out of town to girlfriend's sorority dance and his mom asked me to press it even though it was supposed to be wrinkle free. You know how that goes, I was trying so hard to make it just right! Oh, well, in the end it was fine, repressed without any kind of spray! looked just fine! Best Press is our go-to really good product usually and I think it was the shirt.