From Ampex Service Bulletin 5-02, dated November 18, 
1959, page 2.

"In time, the capstan may become polished by the tape; thus allowing 
sufficient slippage to reduce the speed of the tape by as much as 1%. In lieu 
of factory sandblasting facilities, this slippage may be corrected by 
roughening the capstan with emery paper (Minnesota Mining and Manufacturing 
Company #320 grit or equivalent), so that the scratches lie parallel to the 
axis of the shaft. It is very easy to get a smooth, satiny roughness in this 
manner. However, care must be taken to prevent the particles of the emery 
paper from getting into the capstan bearing. It is recommended that the tape 
transport be placed so that the capstan shaft is parallel to the floor. The 
capstan shaft opening must be covered to prevent the emery particles from 
reaching the capstan bearing."

The above service bulletin applied specifically to the model 300, 350, 400A 
and 450 (and, by logical extension, to every other Ampex model with the 
exception of the ATR series).