I’ve never deployed un-shellac-ed cork grips, so I can’t speak to the relative merits of coated vs uncoated. Just seeing/feeling real cork grips makes me think they *need* coating.
I don’t know about the merits of varnish vs shellac.
I’ve used shellac, two good coats applied an hour apart, with very good results. Spray vs brush has made no detectable difference. Good results meaning the grips are reasonably grippy, not slippery, and they’ve put up with 100s (1000s maybe?) of miles without coming apart. Freezing cold, hot storms. And they’ve survived a few minor scrapings, draggings, bumpings by showing scratches or even a small gouge without completely self-destructing. (I put another coat of shellac on after the gouging.) They aren’t intended as armor, so I wouldn’t expect them to withstand abuse well.
I prefer and use clear shellac though I don’t know why amber wouldn’t work just as well.
When I glue the grips on I use 3m’s 90 adhesive spray. Both inside the grips and on the bars. Expect to want to clean the bars with acetone or something similar afterward. Let them cure 24 hours. The 3m 77 did not adhere well enough. The 90 has been rock solid.
I was able to re-use one pair of grips glued on with 90, with ocd-level efforts to remove them, involving alcohol, acetone, and probably the shortening of my life through chemical absorption and stress. Once. Never again. Generally, I consider glued-on cork grips as single-use only. Want to change grips later? Start with a knife.
Yours,
they get dirty when you don't shellac.