Common Stains and Discolorations

There are many interpretations of the words “stain” or “discoloration” in the wood flooring industry. The definition of stain taken from the USDA Wood Handbook is “a discoloration in wood that may be caused by such diverse agencies as microorganisms, metals, or chemicals. The term also applies to materials used to impart color to wood.” A simple definition of discoloration from Merriam-Webster is “a spot that is changed in color.”

Here I will attempt to clarify and distinguish many of the unique differences between some of the stains and discolorations we often see in our industry. Understanding how to identify and use each term properly should help you and your customers in proper identification of each issue, as well as how or whether it is necessary to address it.

Mineral Streaks/Deposits
Mineral streaks/deposits are a discolored line, spot, or band in the wood that may range in color from light gray to tan, olive green, brown, purple, blue, or black. Mineral streaks/deposits are naturally occurring, and they are often considered to add value and visual appeal to the wood. Many of these stains are merely mineral deposits from the surrounding soil, such as limestone, sulfur, nitrogen, phosphorus, potassium, silica, gypsum, or any other mineral found in the region where the tree was grown.

Mineral streaks are from natural interactions between the living tree and its environment. These are allowed in many flooring grades and will not sand out. No two trees from the same species are identical, no two boards from the same tree are identical, and color, grain patterns, and unique characteristics can drastically vary even within one individual plank of wood.

Sap Stains (Blue-Stain or Brown-Stain)
Sap stains are a discoloration in the sapwood. These stains normally are caused by the growth of certain fungi on the surface and in the interior of the wood when the moisture content and surrounding conditions are prime for this fungal growth. The wood may become prone to this fungi in standing or fallen timber, or during seasoning before the kiln-drying processes.

This discoloration is often known as blue-stain or brown-stain due to the color of the fungi. In kiln-dried lumber, this fungus is not living, does not cause decay, is not a mold, will not spread, is not a health concern, and will not affect the structural integrity or the strength of the wood.

Note that sapwood is the active wood near the outside of the living tree. It is often lighter in color than the heartwood of the tree. This color variation is normal and acceptable in all grades, and must not be confused with sap stain.

Sticker Stains (Shadow)
Sticker stain can sometimes appear as a discoloration on the face of a board associated with the location of the stacker sticker. Stickers are thin strips or boards used to separate the layers of lumber stacked in a pile to permit air circulation during the seasoning of lumber. These stickers can leave a brown or blue stain that develops in seasoning lumber where it has been in direct contact with the boards. Sticker stain discoloration is the result of naturally occurring chemicals within the wood (drying wood and stickers) that occurs during the drying process. These stains cannot be sanded out and are permissible in many flooring grades.

Moisture Stains

Wood or finish can change color with exposure to moisture. These are the stains we have all seen anytime we deal with moisture-related issues. They may include moisture-induced rust stains; stains from plant pots; pet feces and urine; mildew-related, decay-related, fungus/mold-related stains; and many more. If moisture caused the stain, it would be considered a moisture stain.

Iron Stains
Iron stains can occur as a result of a chemical reaction between wood tannins, water, and iron. These stains are most common when filings from scrapers or some types of abrasive minerals have not adequately been removed from the floor before application of water or water-based finish. They can also become apparent from flooring fasteners, metal buckets, or objects that have been placed on the floor with exposure to moisture.

Tannic Acid Discoloration (Tannin-Pull)
Tannic acid includes various naturally occurring, soluble, astringent, complex phenolic substances found in trees (and plants) as a way of protecting the tree from insects, fire, and bacteria. High levels of tannic acid present in woods like oak, walnut, and mahogany can produce a dark discoloration when coming in contact with some products used in the finishing and maintenance of wood floors.

Tannic acid is water soluble and may discolor when it comes in contact with iron and water, which usually appears as a bluish/gray discoloration on the wood. Tannic acid also discolors when it comes in contact with materials that are alkaline by nature, such as ammonia. Most water-based finishes are manufactured with pH adjusters (such as ammonia), which can cause a green/brown discoloration on the surface of the wood, and may also partially bleed into the finish. Reactive conditioners that react with these tannic acids also are commonly used in a controlled method to achieve the desired color or base color in layered systems.

Chemical Stains
Chemical stains are irregular-shaped spots or discolorations on areas of the finished floor caused by reactive chemicals and air pollution. These are commonly caused by reactions with the wood floor finish from household chemical spills or cleaning products such as nail polish remover, ammonia, chlorine, mustard, iodine, oil, milk, ethyl alcohol, acetic acid, etc.

Wood Discoloration
The wood itself changes color over time (darkens or lightens.) Wood changes color through oxidation and photochemical exposure, which is a change that cannot be prevented.
This is a naturally occurring phenomenon. All woods will change color, but some more drastically than others. Some woods darken with age, such as American cherry, Brazilian cherry (jatoba), douglas fir, and purple heart. Some woods lighten with age, such as black walnut and cork. However, not all boards will change color to the same extent.

Finish/Colorant Discoloration
Sometimes the finish or colorant systems can change color. Many film-forming finishes, natural penetrating oils, waxes, and colorants may amber/yellow, lighten, or patina in appearance over time.

This is normal and to be expected. Direct exposure to light may change the color of finishes or colorants over time. This is a natural change and should be taken into account when selecting flooring, finishes, and color systems. Some colorants (such as aniline dyes) and reactive conditioners are not colorfast, meaning they may fade when exposed to light.

Blotchy Stain/Colorant
Blotchy stain or colorant is when there is the appearance of an inconsistent color from one area of a finished floor to another. Often this is simply a physical property of the wood species itself (such as maple, black cherry, and pine), whereby it does not evenly accept stain or colorant. Stained/colored floors should be uniform in color.

Color tones may vary from board to board or within individual boards due to natural variances within the wood. Other times, the cause of blotchy stain is due to not following NWFA Sand and Finish Guidelines or the manufacturer’s instructions. Examples may include a poor sand job, poor stain application, incompatible products, not allowing adequate dry-time, or many other potential causes.

Picture Framing (“Halo”)
When the edges of the room appear to be a slightly different color than the rest of the room, this is known as picture framing or halo. This is almost always caused by improper sanding. Typically, it results from sanding the edges of the room with an abrasive grit different from what the field was sanded with, and inadequately blending the scratch pattern of both the perimeter of the floor with the field of the floor in the final sanding stages.

Filler Color Variation
The wood filler from one area of the floor can be different from the overall color tone of the floor. The drastic variations in many wood species will not always match the filler color used. For example, ash/maple/pine filler will not match all ash, maple, or pine wood. With color-sensitive wood species, the normal filler doesn’t change color with the wood. This is why we normally recommend using the dust from these species mixed with a recommended binder to fill color-sensitive species. Wood filler will never perfectly match, nor is it designed to match every board within a wood floor.

Although stains are technically discolorations that can be clearly distinguished, it doesn’t mean they are always bad or considered problems. When the stain has been produced by hand, it can and likely should be repaired. However, if the stains are produced naturally, they could easily be considered rare and valuable. The interpretation of stain or discoloration lies in the eyes of the wood flooring professional and his or her customer. Making an educated determination of what kind of stain or discoloration is present allows for accuracy in addressing it, or charging more for it.

Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at

5 thoughts

  1. Can’t seem to remove normal type of marks from y hardwood floors. I.E. footprints, shoe prints, paw prints etc. Have been using a vinager and water solution but it doesn’t seem to penetrate. You can still see the marks. Why? And what would you suggest? Thank you.

  2. ours have gotten bigger and are under a window . we just had new windows out in amd they didn’t see anything off . who do we call to check this hit or can we do it by ourself

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