As a former electronics tech we used pencil erasers for corrosion, q-tips and rubbing alcohol for dirt and residues. Removing tarnish from copper and brass connections may require additional chemicals. In that situation I would possibly use brasso and a q-tip for removing the tarnish and alcohol and a q-tip or cotton ball for cleaning up the brasso afterwards. For my vape gear, alcohol and cotton has been all I've needed so far for cleaning. Some of my 510 connections that involve brass get a very light application of dielectric grease after cleaning to slow down the disintegration of soft brass threads.