methods of cleaning old builing reinforcement

I have seen electric used to knock off the old concrete and air hammers. We learned to have the extra guy cleaning them as soon as possible.

wire brush is common, assuming the corrosion is not too extensive and severely impairs the strength of the reinforcement. You can also acid wash cement and rust off the steel, but then you need to neutralize the cleaned surfaces and treat to inhibit further corrosion.

how to neutralize the acid washed steel ?

lime works well to neutralize the acidity on the surface. Cement would neutralize the acid, but this reaction weakens the bonding capability of the cement at the surface of the steel reinforcement.