I know this has been addressed before; What can you apply to a sheet of decals that crack and fall apart after dipped in water?
Decal sealer.
Microscale makes a liquid decal film that is applied and allowed to cure before dipping the deal into water.
Since I buy kits occasionally at flea markets and shows- some quite old- I have gotten into the habit of scanning decals from the kit before trying to apply any. That way, if the decals are bad, and even if a decal preservative doesn’t work, I may be able to reproduce them on inkjet decal paper. Doing dark colored decals is pretty easy, but light colored ones or white ones are a real pain.