Best Way to Preserve Waterslide Decals?

I have a couple of what appears to be water slide decals on my OT 48yr old bicycle.
What would be the best way to keep them from further deterioration?
Not interested in removing and installing repops.
Sorry if it's OT, but I figure the collective knowledge here is second to none!
Thanks in advance.

 

Your best bet is a clear finish that protects the decal from the air and all the junk that is in it including moisture.
The hard part is using something that won't either attack or harm the original decal.
Some of the older decals were not make with water proof inks so any moisture can sometimes make the inks start to run.

I have used Future Floor Polish which is a liquid in the past on some water transfer decals with some success.

Best of Luck.
Jimbo