Technical Data for Stains
Weather-Bos formulas are applied to exterior wood with two objectives: to provide an aesthetically pleasing appearance and protection from the deteriorating effects of the exterior environment. With time discolorations often occur causing owner dissatisfaction and complaints. These discolorations may result from a variety of causes depending on the degree of susceptibility to extractive, mildew, iron and other discolorations. Susceptibility depends on many factors such as surface, climatic, preparation and application conditions. Correction of the discoloration requires identification of the causal factors. The following data will help classify the type of stains which may occur and will provide simple procedures for identification and solutions to these problems.
Classification of stains
Extractive: Extractives stains are the result of naturally occurring chemicals in wood. They give the different species many of their special characteristics. The woods that give their best performance in exterior applications, i.e., redwood and cedar, owe their performance substantially to the extractives which are present. Extractive stains that occur are of two types, those that result from movement of water soluble materials and those requiring non polar solvents for movement.
The most common discolorations are naturally occurring pigmented compounds. These stains are most pronounced in darker woods such as cedar or redwood, but are also frequent in pine, douglas fir and similar lighter toned woods, especially plywood painted with water-based products.
Extractive stains are often called bleeding, referring to discolorations which result when the extractives are dissolved and then migrate to the surface where they are left as the carrier evaporates. The resulting stain can discolor the surface of the unfinished wood, and in many cases create permanent discoloration below the surface. Carriers may come from sources such as high humidity, fog, rain or the natural ingredients contained within the wood. Additional sources of carrier solvent may come from a water repellent preservative treatment, or the organic solvents contained in finishes.
There is also occasional bleeding from the natural migration of resin or pitch, which may appear on the surface as beads of clear or colored resin exudate. This generally is limited to woods dried at temperatures too low to evaporate lighter components of the pitch. It is common in resinous species such as pines and douglas fir, and other soft woods especially around any wound areas.
Resin or pitch bleeding typically occurs on surfaces warmed by sunlight or other sources of heat. Each bead or streak is from an individual resin canal and will appear as scattered discrete deposits. Stains appearing in larger areas may be the result of wounds that occurred in the living tree. Pitch exudations can be removed by using mineral spirits, however there is a strong possibility that pitch exudation will reoccur.
Mildew: Gray to black discoloration is usually caused by the spores of a superficial mold growth. In early stages, it appears as black dots at the original infection sites with a lighter gray fan spreading below. Mildew can colonize on almost any surface not cleaned and maintained on a regular basis. Discolorations from mildew growth, iron stains and dirt are often easily confused.
Tannin Stains: Naturally occurring tannin compounds in all species of wood are in a large part responsible for its beauty, stability and durability; however, sometimes these water soluble tannin compounds migrate to the wood's surface. A high concentration of these compounds may cause a discoloration referred to as tannin bleeding or stains. Tannin bleeding or stains appear in several forms. Some boards, high in tannin content, may turn the wood very dark if unprotected and exposed to moisture. Tannin compounds may bleed around pressure points such as nails, strapping marks or where water is able to penetrate the surface, especially wherever condensation forms. The dark stains below nails on wood siding are the result of a concentration of tannin compounds drawn down by gravity . On the windy side of a structure, the stain line may actually be from vertical to horizontal in appearance.
Iron Stains: These stains are, in reality extractive stains which result from a chemical reaction between the extractives and iron. Most frequently, iron stains are an inky blue-black color with a distribution dependent upon the type of iron exposure. The color is more intense on some wood, especially redwood, cedar and pine, but can occur to some degree on almost any wood. A striking rust red stain can appear on iron contaminated cedar under humid conditions. This results from a reaction with a major extractive component of cedar called thujaplicin.
Other Stains: Bluestain and brownstain are seldom the cause of complaints after applying Weather-Bos, however this is completely dependent upon proper preparation and application procedures. To receive long term protection and pleasing aesthetics requires using adequate quantity of Weather-Bos.
Bluestain is biological in origin and the Bluestain fungi feed primarily on starch stored in the ray cells of the sapwood. The color is usually, but not always blue and is caused by pigment in the mold strands or hyphae. Bluestain molds do not reduce wood strength. Most common to pine, these stains may occur prior to drying, but can also develop in once dry but rewetted woods. They often develop in the interior of a bright board and will be exposed by planing or resawing.
Brownstain is a light to deep brown biochemical stain, which appears during drying but result from prior enzyme activity during storage and processing of logs and lumber. Removal of bluestain and brownstain is not practical since both types occur at depth in the wood, rather than being confined just to the surface.
Most stains are opaque only where the deposits are thick. Generally, they are translucent, in various shades of red, yellow, and brown. Mildew spores are globular and often occur in groups. They are black but as the depth of focus is changed, the interior cell walls can be seen. Spores usually are two celled but four celled and one celled spores are also seen. Frequently, more than one type of discoloration is present. Extractive deposits often form a sticky surface that can trap dirt particles. Stains generally occur because moisture has been present which serves as a food source for mildew or spora growth. It is common for mildew spores to occur along with extractive discolorations and dirt, however one type will dominate.
Always follow Weather-Bos instructions of proper preparation and application. For best results, always power wash surface with a minimum of 2500 P.S.I. to assure maximum penetration. This includes all new as well as older surfaces. To obtain long term protection and pleasing aesthetics requires adequate quantity and proper choice of Weather-Bos along with regular scheduled maintenance.