When cleaning old guns parts were there is a build-up of the kind of varnish from old oil etc, I use the method handed down to me by a very experienced gunmaker.
I simply put all the pieces in a stainless steel saucepan, cover in tap water, add a suitable amount of dishwasher powder or liquid and boil for 10 minutes.
Rinse off with clean water, dry in a warm oven or blow off with compressed air.
De-water with WD40 or similar if any doubts about whether all moisture has been removed, and then oil/grease.
This will remove all traces of old oil and grease from crevices and engraving. It will not harm any existing finishes that I have come across.
A light work-over with a dremel may be required for surface rust, use on coloured surfaces at your own risk!
