Use Acirex sandpaper. I think yellow No. 800 will be clean enough, but if not, polish with light blue No. 600.
Basically, clean the putty with alcohols sometimes before it hardens.
If it hardens completely, soak it in thinner to soak it and remove it.
If the putty has hardened only a little, polish it with Acilex.
That’s about it.