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Benefits of Factory Finishing

Prepriming perfects wood siding

Wood siding has always had a place in high-end and historic homes, and for the purists who simply want a beautiful result on their dwelling. Vinyl siding was, and fiber-cement lap siding now is, the first choice among those who want a low-cost, low-maintenance alternative. The missing message is that wood siding is low-maintenance and does not need to belong only to the upper reaches of the home building food chain. Why? Because over the course of 20 years the overall cost gap between these products has narrowed considerably.

Many of us would pay the extra money for wood siding if we knew that it would not need repainting for a very long time. Lucky for us, there is actually government research that addresses this issue. After all, the machine finishing industry claims that wood is low-maintenance if coated properly. What may be surprising is that "coating it properly" doesn't mean knowing much of anything about how to coat it properly. All you have to do is preprime the wood before installation and it will last. Even prepriming with average coatings would outlast great coatings that are applied only after installation.

Coatings research led by Sam Williams at the Forest Products Laboratory in Madison, Wi., part of the U.S. Forest Service, can back that up with scientific fact.

How does this work? Sunlight degrades raw wood. It breaks the lignon that bonds the wood fibers togetber leaving a surface that is unsound for a tight paint film bond. The photo below shows how this affects the long-term integrity of the paint film.

PRIMED after installation, 20-year-old test fence at the
Forest Products Laboratory in Madison, Wi., is showing its age.

Each column of siding shown has been primed with the same oil-based primer and the same latex topcoat. Furthermore, they were coated 20 years ago within four months of each other.

20 Years Later
Prior to Installation
Weathered for 16 Weeks
Prior to Application of Primer

The one that was preprimed still looks great today. The other was allowed to weather for 16 weeks before the primer was applied. Note that as little as one week of pre-weathering can make the difference.

Other testing at the FPL over the years has led to the basic notion that the perfect coating system for exterior-use wood is one coat of preprimed oil-based primer and two acrylic latex topcoats either machine-finished or hand- applied. My own experience in the lumber and coatings industry has taught me that the natural beauty of real wood can be enjoyed by anyone willing to maintain their siding as often as they might their roof.

The alternative siding choices are not "no" maintenance and all have some form of wear-out feature. Therefore, if you want low maintenance as well as the look and charm of real wood, the only product that has those benefits is real wood.

By Dennis Connelly
Chief operating officer, PrimeTech
Member, Joint Coatings/Forest Products Committee,
U.S. Forest Products Laboratory