With bubbles like on the green frame above, I remove the bubbled or flaking paint, remove the rust with a small wire brush, and then touch-up the paint. If you need to mix paints, the little jars of Testor's oil-based enamel work well. If the whole frame is bubbled, then it's a total re-paint. But for small, spotty areas, touch-up can work fine if you take your time.