The ravages of time and use of a piece of furniture can be limited by a durable top or finish coat. While the terms varnish, polyurethane, lacquer and shellac are commonly used to reference a final finish in general, these products are not the same, nor are they recommended to be used interchangeably.
This finish is actually a natural product (it's made from combining a secretion from the female lac bug with a solvent such as alcohol) that is very safe once dried and hardened. In addition to adding a protective coat, it also can add a warm amber color to wood. It can be affected by heat (white rings will appear under a hot bowl or mug) or chemicals, so a kitchen table might not be the best place to use it. Fine furniture items can be greatly enhanced with shellac. Some shellac manufacturers recommend using it as a protective coat on non-wood items. Apply it with a natural bristle brush or with a cotton rag.
Shellac is available in most home centers as a liquid in a can. It also comes in solid form or in flakes that must be dissolved, and it has a shorter shelf life than other finishes. The liquid variety is the best option for the average homeowner.
Essentially a plastic in the form of a liquid until it dries, polyurethane is available in both water- and oil-based options, and comes in varieties from satin to glossy.
Water-based polyurethane is popular because of its low odor and low toxicity. It goes on clear without adding a slight color that oil-based versions can, and it dries much faster. As with shellac, water-based polyurethane won't hold up well to heat and chemicals. It's good for bookcases, desks, side tables and picture frames — anything that won't be exposed to extremes.
Minwax Polycrylic is an example of a fortified water-based polyurethane than can stand up a bit better to rough conditions. It also can go over oil-based finishes and can be applied using synthetic-bristle brushes, a foam roller or a rag, as can other water-based polyurethanes. Water-based oil-modified polyurethane is a relatively new product that combines the durability of an oil base with the cleanup of a water base. This product can actually be used on wooden floors.
Oil-based polyurethane is slightly more durable than water-based, especially when it comes to handling heat, so a kitchen table is a good candidate. It adds a slight color tone and will bring out the richness of wood.
When working with oil-based polyurethane, use a respirator in a well-ventilated area. Apply using a natural-bristle brush or rag. Oil-based takes much longer to dry and cure than water-based, so plan accordingly and follow the manufacturer's directions.
Both oil- and water-based polyurethane can be applied to latex/acrylic paint; however, oil-based polyurethane will create a yellow or amber hue, especially to light colors. To add durability without affected color, use a water-based finish.
You can also purchase polyurethane in a spray can which makes it a bit easier to apply, especially on large projects. Wipe-on polyurethane is used primarily by woodworkers who want to create a “hand-rubbed” finish on special projects. These two run the extremes of ease of use, but produce excellent results.