Welcome Jim!
I'd tell you to run away while you still can but it's too late for that.
Crocee has you pointed in the right direction. Also take the time to read the sticky on cleaning these old machines and be sure to watch Muv's videos.
Shellac is the clear coat on your 15-91. It dissolves in alcohol. Glenn's tutorial will help you with that.
Some general advice, use sewing machine oil to clean and lube these machines. 3 in 1 oil is too thick and avoid products like WD-40. If you do use a penetrating oil, clean it all off thoroughly afterward and lube with sewing machine oil. Also avoid water based cleaners and strong solvents to protect your finish and decals.
Pictures of your machines and your hats are both welcome.
Rodney