How Penetrating Stains Work

• The penetrating stains commonly used on interior wood consist of a pigment or dye that provides the actual color, coupled with a blend of oils and solvents that allow the stain to penetrate beneath the wood's surface and distribute the colorant evenly. These stains do not seal the wood's surface or offer very much protection from scratches or dings, so most interior wood is also finished with one or more coats of a clear varnish.
  
  

How Gel Stains Work

• Water-based gel stains form an impermeable translucent skin over the wood. This type of stain has become popular in recent years because it emits less odor and air pollution when it's applied. They are also simpler to use, in part because gel stains add a uniform color over the natural wood grain. But gel stains are not tough enough to resist scratching and abrasion, so these coatings require a varnish coat for protection. Many stain manufacturers recommend two coats of water-based polyurethane.
  
  

How Exterior Stains Work

• Semi-transparent exterior stains are quite similar to interior penetrating stains, but they also contain chemical additives that protect the exposed wood from mold and mildew, rot, and other weather-related dangers. In addition, the better grades of exterior stains will contain additives that will prevent the stain from fading prematurely due to direct UV exposure. Those chemical additives are gradually out-gassed into the air, which is relatively harmless outdoors but may be unpleasant in enclosed indoor rooms.
  
  

Applying Exterior Stains Over Penetrating Stains

• When exterior stains are applied over wood that has been treated with interior penetrating stains, the exterior stains will usually darken the original color, even if a lighter color is applied. Over time, both stains will fade from exposure to bright sunlight. An exterior stain cannot be applied over varnish, however. The varnish or polyurethane sealer must first be removed using a chemical paint stripper. The stripper should not alter the color of the existing wood stain.
  
  

Exterior Stains and Gel Stains

• Exterior stains will simply not penetrate through a gel stain. The gel stain must be chemically stripped off the wood, together with the wood's varnish coat. After stripping, the wood will appear completely unstained and natural, and it will easily accept the exterior stain.
  
  

Indoors and Out

• Interior stains do not provide the weather resistance to provide lasting color or protection to outdoor siding, decks or fences. Conversely, exterior stains are inappropriate for interior use because they lack the scuff and abrasion resistance expected for indoor floors, cabinets and paneling. But when unvarnished wooden furnishings are ready to make the transition from indoor to outdoor use, it is often safe and practical to add an exterior stain over an existing interior stain.