Many questions arise when deciding to put wood siding on a new house: “What’s the best type of wood?” “Should I use paint or stain?” “Do I apply finish before or after installation?”
There are no simple answers to these questions, but a recently revised FinishLine factsheet, Solid-Colored Stains on Western Redcedar and Redwood Siding, offers suggestions to help you make informed decisions.
This factsheet focuses on western redcedar and redwood, as these species have long been chosen for exterior wood siding because of their natural resistance to decay as well as their ability to maintain a good finish. Included are tips on choosing the right grade of siding, choosing the right kind of finish, applying the finish, and installing the siding.
Since the mid-1980s, a condition called ‘mill glaze’ (also called planer’s glaze) has sometimes been blamed for the failure of a coating on smooth flat-grained siding and some other wood products. The exact cause of this problem has been a subject of controversy. This newly revised Finishline tackles the tough question of Mill Glaze: Myth or Reality?
Many people believe that the coating fails as a result of the planing and/or drying processes. They speculate that the milling or planing process overheats the wood and brings more water-soluble extractives to the surface, creating a hard varnishlike glaze. They attribute overheating to dull planer blades.
An earlier FinishLine (‘Why House Paint Fails‘) described the problem of mill glaze. Tests trying to duplicate mill glaze failure in the laboratory have been inconclusive. Although research on ‘mill glaze’ effect has not continued at FPL, a number of reported mill glaze failures have been reviewed. In all cases, the failures were readily explained by other failure mechanisms, including:
- Raised grain,
- Degradation of the wood surface by ultraviolet (UV) radiation prior to painting,
- Insufficient thickness of the coating system,
- Improper surface preparation, and
- Moisture problems.
Iron stain, an unsightly blue-black or gray discoloration, can occur on nearly all woods. Oak, redwood, cypress, and cedar are particularly prone to iron stain because they contain large amounts of tannin-like extractives. To find out more about iron stain, and what can be done about it, click on over to this newly revised FinishLine.
The discoloration from iron is caused by a chemical reaction between extractives in the wood and iron in steel products, such as nails, screws, and other fasteners. Contaminating wood is easy. For example, merely striking wood with a hammer can cause iron stain on some wood. (Covering the head of the hammer when nailing redwood and western redcedar siding is a good idea.)
Wood left exposed to the elements can last for centuries. This newly revised FinishLine explains how the type of wood and how it’s protected can make a big difference in how long the wood lasts. Two conditions influence the service life of outdoor wood: weathering and decay.
Tips for prolonging the life of wood finish include:
- Subject the wood to one or two wetting/drying cycles before you apply the finish; wet the wood (with a hose, for example) and let it dry completely each time.
- Sand the wood before applying the finish.
- Apply paint to wood soon after sanding. If possible, paint in a protected area, such as a garage, to protect the wood from the sun. Even 1 week of sunlight can weather unfinished wood and shorten the life of the paint. See the FinishLine “Before You Install Exterior Wood-Based Siding” for more information about sunlight on wood.
For more information on how to finish outdoor furniture, artwork, fences, and play equipment, see the following FinishLines: “Paint, Stain, Varnish, or Preservative? It’s Your Choice” and “Finishes for Wood Decks.”
When acids come into contact with unpainted wood surfaces it speeds up deterioration and can also affect the performance of paint applied to weathered wood. A newly revised FinishLine, Effects of Acid Deposition on Wood, explains why.
In tests conducted near Madison, Wisconsin, smooth-planed wood was allowed to weather before painting. Exposure for as little as two weeks shortened the service life of the subsequently applied paint. The paint bond was weak and the paint eventually peeled. Acid concentration in rain near Madison tends to be much lower than that found in many other areas of the United States. High acid concentration would tend to produce more surface degradation and thus form a weaker paint bond. In any case, wood should be painted as soon as possible after it is installed outdoors.