The slides are a mix of Kodachrome and Ectachrome. The dirt is dust specs and very small hairs. The slides are 1-2 decades old. They have been mounted in regular cardboard mounts and stored in projector (Bell and Howell) plastic cubes.
Okay, the Kodachrome ones are the hardest because they can only be cleaned mechanically. I use oil free compressed air (from an airbrush compressor or suitable propellant free dry compressed air can) for the dust removal. Then if there is still fused dirt, the Kodachrome ones may need rinsing in distilled water and after applying a final rinse in a photoghraphic "wetting" solution (to prevent water drops), drying in a dust free environment.
The
non-Kodachrome ones can be scanned with a scanner that has some form of "Digital ICE" IR dust/scratch removal capability. It's a life saver if you have a lot of slides that need repair.
If it is grease/finger prints that need to be removed, there are dedicated cleaners, but I am very careful in avoiding any rubbing since it could scratch the emulsion even more. That's why it is prefered to first try ICE.
Older sildes can also suffer from discoloration/fading of one or more color layers, there is software that attempts to reverse the fading process in postprocessing, but with variable success.
Bart