Why Back-Priming and Pre-Application are not Required with the Traditional Coatings.
Do I need to back-prime and therefore apply my traditional Authentic Pine Tar finish before installation of exterior siding?
This is a very common question and the simple answer is "no."
While back-priming and pre-application have become an industry standard in North America, these are relatively recent practices and based on the use of modern, petrochemical-based coatings.
Modern coatings primarily rely on an encapsulation strategy or application on all six sides of timber siding in order to create a protective seal. This can work in the short-term but wood moves based on seasonal changes, quickly creating hairline cracks in the coating that allow moisture to get inside. This relationship leads to extreme failure that generally requires extensive scraping and sanding to return the wood to a state where it can be re-stained or re-painted.
The traditional coatings, including Authentic Pine Tar, rely on a capillary effect, meaning the natural oils soak in and absorb into the porous wood substrate, traveling through the wood over time vs. sitting on the surface like a hard, plastic shell.
This is a very different type of relationship between materials in comparison to complex petrochemical formulations and a reason why the traditional, natural oil-based finishes work so well and offer much easier, less labor intensive maintenance in the years to come.
So, it is not necessary to back-prime or pre-apply a Pine Tar finish. The easiest set of steps would be to install the wood siding and apply two thin coats minimum of the Pine Tar finish in place, following a right to left, top to bottom (or similar) application, following full board length to ensure an even, consistent finish across the facade.
If the wood is kiln dried or exposed to excessive standing moisture, like a deck or a shake roof for example, then additional coats can be very helpful in terms of providing additional protection.
While not required, many projects still back-prime and pre-apply the Purified Linseed Oil or the Authentic Pine Tar mixture.
Below is a exploration of possible benefits and suggestions based on customer feedback from projects in various North American climates.
Possible Advantages of Back-Priming
Most wood in North America is incredibly dry, especially if it is kiln dried. So, a possible advantage to back-priming would be that a coat of the Purified Linseed Oil or Pine Tar mixture would assist in offsetting any natural cupping or warping of the boards after installation.
Wood moves and sometimes it moves dramatically based on quality and dryness, wood species, milling/processing methods, moisture content, etc.
This potential benefit is difficult to quantify, meaning an attempt to stabilize super dry wood with more natural oils before installation may or may not make a huge difference. It really comes down to the integrity of the wood to begin with i.e. cheaper, drier wood will generally be less stable than wood with more natural resins, which is generally a higher quality timber.
If these possible advantages seem worthwhile, we would recommend you consider just applying the Purified Linseed Oil or the Purified Linseed Oil and a small percentage of Pine Tar (for it's insect deterrent properties, which could be beneficial with a rain screen design for example) on the back of the boards. Not only would this be economical, as there is no need for UV protection on the back of the boards, it would also achieve a faster dry time as the Purified Linseed Oil dries faster than the naturally more resinous Pine Tar.
Possible Advantages of Back-Priming
Assist in offsetting cupping or warping.
Improve overall quality of timber quickly.
Additional moisture and insect deterrent properties.
Possible Challenges of Back-Priming
If back-priming, there will be an additional expense as the square footage is essentially doubled by coating both front and back sides of the boards.
If you back-prime and/or pre-apply it is essential to have a system in place so the boards are able to dry fully before storage and stacking, transportation, or installation.
Do not attempt to install damp, wet boards. This will make a mess.
The boards can chalk a bit, this is natural within a 1-2 week timeframe after application, but they should feel fully dry to the touch before installation.
Do not add a drying agent or additive to the Pine Tar to speed dry time.
Pine Tar has the longest dry time, 72 hrs between coats in ideal 60+ degree temps, of the natural coatings and it is a part of the reason it is such a resilient, long-lasting exterior finish for wood surfaces. Embrace how and why this material combination works so well.
If a purified/degummed/cleaned linseed oil is used, like the Viking Purified Linseed Oil that we suggest in combination with the Authentic Pine Tar options, no additional solvents are required. The use of a real gum turpentine (not a conventional turpentine or mineral spirits, which are petrochemical based) has historic precedence but it does not contribute any real benefit to the wood like a Purified Linseed Oil. For health reasons, any additional solvents should be avoided whenever possible.
Challenges of Back-Priming and Pre-Application
Increases overall material costs.
Space and storage considerations for drying.
More time required to complete application.
Increased chances for damage or error.
Resins can easily gum up saw blades during install.
Finally, if a project does decide to proceed with back-priming or pre-installation, we highly recommend the application of the final coat after installation.
Boards inevitably get bumped around and cut during install. A final coat once everything is completed will achieve the most consistent finish across the facade vs. risking spot treating damage, end grain, or fasteners.
See general application suggestions article also on the Learning Center and remember, an initial coat can be applied before or at installation and additional coat(s) can be applied in the seasons to come.
General application technique for all finishes, modern or traditional, recommend a right to left, top to bottom (or similar) application strategy to ensure an even, consistent finish across the facade. Avoid applying a finish across the ground level of vertical siding and then across the 2nd story. Follow the full length of the boards, top to bottom for example, to avoid overlap and visual striations, especially with pigmented finishes.
For questions, please contact info@earthandflax.com.
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