Proper preparation of a wood surface is essential to good results when using any clear finishing material. Not only won't that type of finish hide any irregularities and blemishes, it will accentuate them.
This is as true with the old standby, shellac, as it is with varnish, lacquer or any transparent finish, natural or synthetic. Before you apply shellac, be sure the surface of the wood is as smooth and unmarred as you want it to be when the project is completed. Mostly, this involves sanding.
The thinner for shellac is denatured alcohol, the kind known as pure. If you have a four-pound or five-pound cut, it means four or five pounds of shellac gum have been dissolved in each gallon of alcohol. Further thinning is needed for most purposes.
While getting acqainted with shellac, thin it 50-50 with alcohol. As you get familiar with its many uses, you will learn what the proper mixture is for the job at hand. You can't go wrong if you remember that two or three thin coats are better than a single thick one.
Shellac has long been used as an undercoat for varnish, but you have to read the label carefully to find out whether it is suitable. Certain kinds of synthetics, especially polyurethane varnish, don't take kindly to a shellac undercoat. Generally, an undercoat of shellac with regular varnish helps prevent darkening.
Because shellac dries quickly, cover a spot you have missed with the second coat.
Shellac should not be applied when there is high humidity, when it takes longer to dry, and thus is more susceptible to dust. Also, the finish later may take on a cloudy appearance. Should this occur, the cloudiness can be eliminated by going over the surface lightly with a cloth dampened with denatured alcohol.
One of the reasons most people like shellac is because it is glossy. If you find it too glossy, you can cut down the shine by rubbing it with steel wool or powdered pumice and a little oil or water. When it is desired to retain the original gloss, but provide some water resistance and extra durability, apply paste wax.
Some woods, especially pine and cedar, may take stain unevenly. In that case, give the wood one of the wash coats previously mentioned. Mix the shellac with two or three times as much denatured alcohol as shellac. Wait a couple of hours, sand very lightly with fine sandpaper, dust and apply the stain. When you are certain that the stain has dried, which may be overnight, put on another coat of shellac, this time thinned about half with denatured alcohol.