Most of the celluloid that has not been seriously aged can be polished directly with toothpaste. Due to the delicate characteristics of the celluloid material itself, polishing will be quite easy. However, severely aged celluloid and some special models (represented by Sheaffer) , the pattern is synthesized by pressing multiple pieces of celluloid), you need to add a protective layer (such as varnish or wax, etc.) first and then polish it.
Hope this can help you