Fabric-covered surfaces were sprayed with cellulose acetate nitrate dope, originally. Since aircraft factories commonly had their covering shop right next to the welding shop, fires were not uncommon, and the plant managers never seemed to learn.
I question whether big museums like NMUSAF or NASM are so authentic as to use the original materials, or whether safer Butyrate dope (in general use since the 50s) has supplanted nitrate.