I want to switch to wax. For preparation, I am cleaning my cassette, the gears in the bicycle derailleur (and the derailleur frame), and the bicycle chainring (I don’t need to clean the chain because I am getting a new one).
For the job, I got white spirit, isopropyl alcohol, and a brush, and I cleaned them as well as I could, but, for example, on the cassette there are still some black dots left and, when touching them, they are still a little greasy. How clean do the components need to be, and how can I get rid of the leftover grease?
You need to clean the chain really well before the first wax, so that bare metal is exposed for the wax bond. The rest of the stuff you’re talking about (chain ring, cassette, etc) isn’t getting waxed and doesn’t really matter (although now is a fine time to give them a relatively deep clean). The wax lubricates motion among the plates/pins/rollers of the chain, and that’s it.
There is no relative motion to lubricate between the chain components and the sprockets - each time a roller comes into contact with a sprocket for a trip around the gear, it stays fixed in place and the pin rotates inside the roller. This design leads to chain wear (easy and relatively cheap to replace) instead of cassette and chain ring wear (expensive).
Zero Friction Cycling is the place to go to read about waxing details. Here’s their chain prep guide.
Silca now makes a product that promises to make the initial chain cleaning trivially easy, but you need to heat it all to 125°C to work (instead of only 75°C for normal waxing), so to use it you pretty much also need their expensive crock pot with its precise temperature control (normal crock pots don’t get that hot).