1. As WClark has already said, it seems like you need to cure the touch-up paint for a lot longer than the instructions say. Cure it according to the instructions, and most of the touch-up comes off when you do the Langka treatment. I haven't waited "several days" as WClark suggests, but I have found that waiting several hours improves results.

2. Don't expect a miracle. You'll see the best results with colors like black and red. On metallic paints, the metal flakes line up differently when you blob paint on, as opposed to when it's sprayed on at the factory (light is reflected differently), so the touch-up will always be noticeable no matter how "smooth" it is. This isn't a criticism of Langka; it's the nature of the paint. But you need to know that Langka can't work miracles.

3. Some people have suggested using a mild hand glaze, like 3M IHG or One Grand Omega Glaze, <i>instead of</i> Langka, while using the same technique. Glazes cost a whole lot less per oz., plus you have another use for them. Per ounce, Langka is extremely expensive. I haven't tried using IHG as a Langka substitute because I bought Langka before I'd heard this advice. But considering the cost involved, I think it's worth experimenting with glaze on a couple chips first.