The Right Stuff
Shellac is made from the secretions of an insect, and wax is naturally present in the end product. Although the wax doesn't interfere with shellac's ability to adhere to wood or to itself, it can affect the ability of another finish to adhere to the shellac. Unless you know for certain that an old coat of shellac is of the dewaxed variety, the best course of action is to apply a fresh coat of dewaxed shellac over the old. You can buy dewaxed shellac, or make your own by allowing the wax to settle out of a fresh batch and filtering off the clear top layer.
Before applying any finish over another, first make sure the old finish is sound. If the shellac is discolored, murky or sticky, or if there are any chips or cracks, the old shellac should be removed. Luckily, shellac is fairly easy to remove with denatured alcohol. After stripping, or if the finish is sound, clean the piece with mineral spirits, then lightly sand it with 220-grit sandpaper. Wipe well with a tack rag to remove dust.
Shellac can be applied with a pad, brush or sprayer. Remember that shellac dries very quickly, and can be tricky to apply smoothly with a brush. Whichever method you use, test your technique on scrap wood first. Also remember that this is only a thin sealing coat. Let the coat dry thoroughly, then sand it lightly with 320-grit paper. Remove the dust with a tack rag. Apply your finish coat according to the manufacturer's instructions.
Although there is some disagreement as to which wood finishes can be applied over others, most woodworkers agree that a coat of dewaxed shellac can be used underneath almost anything. When in doubt about any refinishing technique, always run tests on scrap wood.