if you are wet sanding between coats there should not be any contaminants... A little soap and water with the wet sanding and you should be fine. The devil of painting does lie within the prep work. From a clean base (wipe down with alcohol and then a good coat of primer), put down your first coat. After the first coat has had a day to dry in a dry place (such as your garage), wet sand to 1600 starting at 800 with a few drops of soap and water. Let it dry, lay down one more coat, rinse and repeat until you have enough coats of how every many you want to lay down. I would not use windex to wipe between coats, if you must use something use alcohol. The alcohol dries off and leaves no residue. Also do not use a regular towel or something of the sort to wipe between coats, use a new microfiber cloth. After all your coats, remember to wetsand then clear then buff and wax.