Brass fill brush - particularly suited jobs where marking or damage needs to be avoided, for example when cleaning electrodes on spark plugs.

Nylon fill brush - will not rot which make is it suitable for engine cleaning and general de-greasing or where copious amounts of cleaning agents are used. 

Stainless Steel Fill brush - the semirigid fill of this brush is more suited to removing debris, rust and generally preparing surfaces prior to painting or welding.  Will not rust when used in conjunction with wet cleaning agents.