PLEASE DO NOT USE SOAP AND WATER TO CLEAN OLD BLACK SEWING MACHINES.
Sorry for shouting. I just don't want you to do something you'll regret afterward.
The old black ones aren't painted. They are covered with japanning. It's a black finish that is closer to varnish in composition with asphaltum added for the black color. They paint it on the surface then bake it on to cure it. After that the decals are applied and then everything gets a clear coat of shellac to protect the decals.
The shellac either wears off over time or becomes old and brittle and flakes off. That leaves the decals unprotected.
exposure to water will either cause the decals to turn silver or destroy them completely.
Sewing machine oil is safe and effective for cleaning old sewing machines and won't harm your decals. That's why we always recommend only that.
There's a very good chance that just oiling and cleaning your machine will free it up to where it can be more than just a decoration again. It takes a lot to kill these things.
Please post some pictures too. We love pictures.
Pushy, aren't I,
Rodney