Originally Posted by Mickey2

Silvered decals only happen when the top layer of shellac wears down, the damage is caused just as much by mechanial friction as any chemical reaction. As you know alcohol containing products are by far the worst on shellac, a slightly damp cloth doesn't do much damage if it doesn't leave any dampness behind. When we clean vintage machine I guess we start with the gentlest of cleaners, then amp it up as needed to get old grime and dirt off the surface.

Silvered decals are very delicate, the only thing I know that helps is to clean very gently with cotton swabs around them, then apply a layer of shellac to protect what ever is left. The old Singers don't have prefectly applied and polished shellac (compared to guitar and violin finishes), but it's done well, nice and shiny. My point is really to gently apply shellac with next to no rubbing until a protecive coating is formed.

With the inside of the machine, I have had just as good results with spray can oils as anything else. It is a bit more expensive, but works. They easily seep out and end up just any where, they probably will affect exposed decals. Shellac in good condition usually stands up to them. Repeated action will dissolve the worst of grime, but I don't know how much time and energy you are willing to put into a machine you plan to sell. Brake cleaners generally contain more hazardous ingredents than they basic WD40 or CRC 5-56, and they dry up much quicker than spray can oils. An effective and economic solvent is white spirit applied strategically with a dropper to internal areas, it is a bit stinky but works on old oily grime.