For the record Humbrol 49 is an enamel varnish which from my experience would never use for anything, especially since it's thinner/solvent can damage expensive decals.
Never happened to me. I have used it for so many years. The only problem I've had with it has been since I returned to the hobby and Humbrol came up with a new "recipe", causing the varnish to get the consistency of jelly after a handful of uses. This is even after cleaning around the lid thoroughly before putting it back on.
The old, good varnishes are orange, almost red as you open the tin. They require no thinning, and no stirring. A little shake is all you need to perform for them to spray beautifully. They are touch-dry within minutes. This even goes for their gloss varnishes. I have found two of these cans in shops since 2013. The rest have been garbage. Old recipe Humbrol varnishes are simply the best, changing the recipe was a huge mistake - but not once have they affected the decals in any way, shape, or form for me.