Toot,
You have recieved some good advice! Just stick with it and the knack will develop.
I might add a few things as well. Check to see which way the shaft is spinning. My experience shows that you can get a thinner line and better control if the shaft is spinning toward you. This means you would hold the brush UNDER the shaft to apply paint. Also make sure the shaft is well supported on the other side of where you are cresting. Like was mentioned before, it is tough to get a good line when most of the shaft is whipping around.
I use the Testors enamal paint as well. I spray on my cap dips with Testors spray enamal cans. One coat of flat white, then it only takes one coat of color (I like the "competition orange" because it matches my orange fletching). As another suggested, when I spray on my flat white, I choke up with the tape to leave the cresting area white.
I have had trouble mixing enamal and acrylic. They just don't work together for me. I have had some problems with mixing waterbased and oil or solvent based, but it seems some of these guys have worked that out (I took some notes for my own use).
I agree with the guys who think your motor may be spinning too fast. Also, hardware store dowels are cheaper than good arrow shafts for practicing on if ya want.
I am no artist by any means but have developed my skill to the point that I'm not embarresed to let buddies see my paint jobs. The deer (or other critters) don't care. Although cap dipping and cresting make arrows look good it can also be functional. Almost all the arrows I shoot now are cap dipped bright orange. I can see where my shot goes much easier. This boosts confidience in a good hit. I can also find the arrow on the ground easier after a miss or pass through. When I shoot 3-d rounds with buddies, everyone knows exactally which arrow is mine because of my personal color design.
And besides... its just real cool! Right?
Good Luck and like one of the others said, have fun. I have included a couple of pictures of some of my arrows I have done. Both woods and carbons. Not art, but at least functional.
OkKeith