Removing the clear layer of shellac from an old Singer sewing machine is possible without causing any damage at all to the paint beneath. Shellac is a clear finish, like varnish, that Singer applied to their machines to protect the paint from damage or staining, as well as to make the task of cleaning the machine a little easier. Shellac can be removed from a Singer sewing machine without damaging the paint, as long as you take care not to scratch the paint during the removal process.
Put on a pair of rubber gloves to protect your hands. Apply denatured alcohol to a 5- by 5-inch section of the sewing machine with a brush. Allow the alcohol to sit on the shellac for a few minutes to soak into the finish. You'll know when it's been enough time by testing the shellac with the scrapper. If the scraper removes the shellac, then it's ready to be wiped away.
Wipe the shellac off the sewing machine section using a rough cloth. Use a small circular motion to remove the shellac.
Repeat the process, removing the shellac in small 5-by-5 sections until you've covered the entire painted surface. Change the cloth used for the removal process with each section to avoid reapplying the dissolved shellac now carried by the cloth.
Examine the finish to determine if the single application of alcohol was sufficient for removal. If any shellac remains, then repeat the removal process.
Run a small soft scraper over the surface of the paint after shellac removal to scrape away any shellac residue. Make sure you do not gouge the paint on the sewing machine with the scraper.
Perform the removal process in a well-ventilated area for safety from fumes.