In my railway model days some 40+ years ago, there was a type of decal that involved alcohol. Stretching memory, "Methfix". Checking memory with the help of google, they still exist, but it appears that today they come with instructions, which would have been nice back then.
Waterslide transfers consist of a layer of strongish paint or similar on a layer of water soluble glue on a paper backing. Applying requires that the glue be wetted and the transfer slid off onto its destination. Since the glue remains soluble, protection is needed, usually some variety of varnish to match the paint already there.