Pesky blister
I really like a couple of coats of shellac as a sealer and haven't had much in the way of problems, but I do have one canoe I used shellac on under varnish and there is a pesky blister in one spot. It may have originally developed in the sun. Alternatively, there may be some sort of contamination in the finish (I hear you should use de-waxed shellac under varnish), but I keep removing the blister, cleaning it with solvents, sanding and revarnishing only to have the blister return, and only in one spot.
I really like a couple of coats of shellac as a sealer and haven't had much in the way of problems, but I do have one canoe I used shellac on under varnish and there is a pesky blister in one spot. It may have originally developed in the sun. Alternatively, there may be some sort of contamination in the finish (I hear you should use de-waxed shellac under varnish), but I keep removing the blister, cleaning it with solvents, sanding and revarnishing only to have the blister return, and only in one spot.