Finishes checklist: a guide to achieving optimum coating performance on exterior wood surfaces

JCT CoatingsTech, March, 2004

When the time comes for a consumer to select the wood and finish types for a given outdoor project, there is a wide variety of sources of information, articles, and opinions available. Occasionally, these sources will conflict, mostly due to the data available at the time of publication, or practical experience based on a snapshot of conditions at a given time period. It is our hope to make this article available as a tool for the consumer to weed through the confusion, obtain the best system for the particular application, and understand the compromises that can result from other choices. A bibliography is included for more in-depth research.

STARTING BASICS

To avoid any problems, the best place to start a project is when the structure/home is originally planned. In this way, one can be assured of correct construction practices, that desired types and grades of wood products and proper selection of treatments and finishes are used, and that they are all properly applied.

In the real world, we are typically not that fortunate. A review of the basics, however, may help identify potential problems and avoid or correct their causes before major corrections or damage may occur.

BUILDING DESIGN BASICS

For best performance of a siding finish, certain design choices should be made. These choices can help prevent moisture accumulation or excessive moisture fluctuation within the building components. They include, but are not limited to:

* Use of dry materials during construction

* Provide adequate clearance to grade and drainage at grade

* Design with adequate roof overhang to protect the siding from weathering.

* Install an interior air barrier and vapor retarder in cold climates

* Install vertical furring strips between sheathing and horizontal lap siding to help with moisture removal

* Install gutter guards to prevent ice dams from occurring during winter months.

* Due to wicking that can occur on the edges of wood siding, it is highly recommended that the contractor seal the ends to help keep moisture from wicking into the wood and causing early coating failure near the end of the board.

* Where possible, the practice of back-priming or sealing all sides of the wood siding can extend the lifetime of the finish substantially.

* Use of an exposed strip of either galvanized or copper flashing at the top of the roof can help prevent mildew growth on shingles in the path of the rain water washdown.

For more on this subject, refer to the Moisture Control Handbook, published by the Oak Ridge National Laboratory, or Finishes for Exterior Woods, by R. Sam Williams, Mark T. Knaebe, and William C. Feist, available from the Forest Products Society.

CHART BASICS

The Surface/Coatings Checklist is divided into several sections, each containing information regarding durability and general recommendations for improving the life expectancy of the coating system over given exterior wood products.

The following text discusses the key features of the checklist and provides some examples.

Wood Type Section

In this section, different type of wood products are listed. There is also a column for footnotes, giving information on how to get the best coating performance.

[ILLUSTRATION OMITTED]

FPL Info Section

Referencing the Surface/Coatings Checklist the different wood species have been ranked by the Forest Products Laboratory for their characteristics of painting and finishing. For paint holding characteristics, the species are rated from best [I] to worst [V], where hardwoods with large pores are ranked [V]. These species, when filled properly with wood filler can hold paint and qualify as grade [II]. Other species not listed in this chart include Douglas Fir [IV], Spruce [III], Yellow-poplar [III], and Oak [V or IV].

They are also rated by resistance to cupping [scale of 1-4, with 1 being best] and by conspicuousness to checking [1 or 2, 1 being best].

In this section, composite woods have not currently been rated, as these are being evaluated.

Coatings

This section consists of six groupings, corresponding to the generic coating types available on the market today, from water-repellant preservative to top line latex house paint.

Each grouping consists of four columns. The first two columns--water repellant use and primer use--show how much additional durability can be expected by using either or both options. The effect is additive. For example, if use of a paintable water repellant gave an additional year of durability to the coating system, and use of a primer gave an additional two years, using both should give an additional three years.

The second two columns--one coat and two coats--each show the general expected average durability of that coating system with the respective amount of applications. For example, one coat of solid color latex stain might give four years durability, but two coats might give seven. Assuming the previous example for water repellants or primers were used, up to an additional three years could be added to that coating system's life expectancy--giving a maximum of 10 years durability.