If it's a commercial table, it could be catalyzed lacquer. But to check if it's regular lacquer, take some lacquer thinner and rub it on a spot that will not show. If it's regular lacquer, the thinner will remove it. It will not remove catalyzed lacquer or poly.
Another thing you can do is just lightly sand the finish. You probably can get away not doing this, but you can next spray on a coat of dewaxed shellac then follow with whatever you want to put on the table. The shellac will adhere to the original finish and your final finish will adhere to the shellac.
But if the orignial finish is 20 years old you can probably just sand and spray without any problems.
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