When I rode an early 50s ex-military Triumph I used to oil a rag and wipe the bike down. It kept the surface rust in check and maintained the original paint nicely.
I had a friend who wanted to clearcoat over the finish on his badly weathered early 70s Suzuki. He gave it a clean, and dried it carefully so the clearcoat could stick properly and then headed in the house to get his rattlecan of clearcoat. Upon returning he found that one of the local seagulls had made a major contribution to the patina on his fueltank.
It coated well, lumps and all, and was still a topic of conversation several years later.
So while I can’t speak for the viability of clearcoat on an old army bike I do know it does a great job on seagull poop.
Hope this helps.