On some older rackets I've come across has a gummy flat finish. Some others have a dry grease finish like on drinking glasses that have been sitting on a shelf for a long time. The later can be removed with alcohol and some elbow grease.
If they are just gonna be demo rackets, you might as will leave them, and replace the grommets if needed.
Duplicolor works great for me on my car parts. A client of mine sanded down to the graphite, and used a clear coat that never really dried properly.
Machines: Gamma 6004 2-point w/ Wise 2086 & Babolat Sensor Dual