How do you spray fine lines (Badger 155 Anthem, Humbrol enamel paint)

If your hand isn't already moving when you hit the trigger and still moving when you let up on the trigger, you'll get those dots at the beginning and end of the line. It's the lack of motion that causes them.

I'm used to painting camo patterns instead of lines, so It will be difficult to do this painting a single line. If you angle the airbrush so that the tip is pointed in toward the color you are painting the overspray will go in that direction and give a sharper line on the other side.

Don't try and get the final coverage on the first pass. Paint the dark color, then load up the lighter color and go back and "Tighten Up" the pattern. Sometimes it takes a couple of iterations between colors to get a good demarcation.

As to the airbrush taking on a life of its own, welcome to my world! I wish I could say mine does exactly what I want it to do, but I'm too much of a rookie and it isn't even close.

Thanks for the reply, maybe I was expecting too much to paint the line in one go. Didn` t even think to respray with the base colour.
Thanks again

It can be done, I'm quite sure, but I can't do it.

Like I said, I primarily do camo so I've got a decent area to work in. The technique you need may be quite different. I lightly draw a dividing line with a pencil and then paint just outside of it. I paint at low pressure (around 8 psi; hard to tell since my gauge barely registers) with pretty thin paint so it will flow. That lets me get real close to the surface, roughly an inch away, without blasting paint everywhere. The low pressure also cuts down on overspray. I outline the area, fill it in, and then go back to the base color and tighten up the demarcation line a bit.