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How to Clean Up and use a vintage sewing machine - videos by Muv and Fav
Basically, just wipe with soft cotton cloth and sewing machine oil. The metal pieces can be removed and polished with metal polish. If the worn places are smooth, can just oil. Some have used other things to clean or polish with mixed results. Some products (even just water) will silver the gold decals so be careful if you should use anything other the sewing machine oil.
As for the treadle base, I have only used sewing machine oil on those. However, if you want to touch-up the parts that are down to bare metal, I used a Testors paint with a fine brush on my sister's machine head. My brother repainted the treadle irons. Another paint that could be used is Duplicolor #DE1635 Spray paint. as indicated at
Let's talk about rust and if just touching up small areas, you can spray into the lid or something and apply with a brush.
Janey - Neat people never make the exciting discoveries I do.