This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A good brush for applying acid to articles for soldering can be made of a piece of small copper pipe for the handle, and fine copper wire for the brush. To make the brush part, take a piece of cardboard, about 1 1/4in. wide, and wind several turns of No. 28 gauge copper wire around it, then remove the coil, insert about 1/2 in. of it in the pipe, and flatten the latter to hold the wire. Clip the ends of the wire, and a brush will be had that ordinary acids will not affect. If only a short piece of pipe is available, it can be used as a ferrule and a handle made of wire or wood. - Contributed by A. R. Cunning, W. New Brighton, N. Y.