I use an oil base paint pen, fine line black. Made by Mitsubishi Pencil, conforms to ASTM D 4236. Its fast drying. I think it comes in a couple different colors. Its actually a little metal cylinder about six inches long and half inch diameter that has a ball bearing inside that slides back and forth similar to an aerosol paint can. Probably same type Anthony is using. Last for years ouside. Some so called paint pens wash off or fade so you have to be cautious when you buy-these are not 'kiddie' pens. Spring loaded valve action tip. Pictures added for visual reference only o5/07/12
