Originally Posted by Duragrouch

In that area inside the seat tube, and the exposed metal on the bottom of the bottom bracket, I would obtain a bottle of Ospho, it is a mild phosphoric acid solution (harmless if you get on your skin, but still wear gloves and glasses), apply with clean "acid brush" (cheap, disposable) or clean toothbrush or just paper towel, let sit for a day, that will (gray) phosphate the surface (a mild version of Parkerizing on military stuff), and that will give the paint better grip. These days, many car parts and body panels will be phosphated before paint. It also replaces primer. Apply second time if needed. Then clean area with alcohol, let dry, and paint. Bike Friday has used powder coating paint for a long time, so I don't think you can get touch-up paint, and for cars, that has gotten ridiculously expensive.