I like shellac for head cement on my trout flies, but that use consumes it verrrry slowly and I've learned that old shellac takes up water from the air. Once it's taken up some critical amount it doesn't dry properly in any amount of time.

I have no idea how long that is. But anyway, try a brand-new can from a store whose stock turns over quickly. Or you can go hard-core and buy shellac flakes from a high-end woodworking outfit and make your own. I think the solvent is just denatured alcohol.

Good luck with it.