Post-pre-treatment and prior to painting or powder coating, the surface may have to be treated with filler materials to cover dents and deformities.

Putty

Putty or spot putty is the main material used for filling the dents. It is a kind of cementing material containing a mix of calcium carbonate powder and linseed oil. There is a variety of putty available in the market for this purpose.

For mild scratches and dents, polyester-based putty is used. It is a one-part compound filler good for many applications except in extreme high-temperature environments.

For deep gashes, metal putty is used. They are normally two compound fillers with polyester or epoxy bases. They are mixed with compounds of metals such as steel, aluminium, or bronze. Putty can be diluted using recommended thinners and is applied with putty blades.
 

Application of putty with blade. (Image source)

Application of putty with blade.

Orbital Sander.

Polyester putty application on auto body.
(Image source)