This is not a better method. Just another one...
I have not had a chance to build a dip-tube so my method of varnish application is wiping it on. I apply 2 coats of varnish before putting on the guides. I feel that it gives the bamboo a layer of protection from the metal guide-feet and trapped water. I use a NON-cotton TShirt (the cotton leaves fibers in the varnish) to apply in long strokes. If you rush, you'll get bubbles so go slowly. When you get a few bubbles, gently blow on them and they'll pop but if you blow too hard it'll move your varnish. I will wait about a week and then wrap the guides afterwhich apply 2 coats of varnish on the wraps with a small art brush.