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When it comes to finishing exterior timber including weatherboards, cladding and screening, surface texture can have a significant effect on performance of semi-transparent coatings.
Smooth Dressed Timber
Dressed face timber suffers from ‘planer glaze’ which is the pressing down and sealing off of the wood cells during processing. This makes the surface very smooth and dense closing cells off to penetration and resulting in a thinner layer of stain being applied. This means pigments can chalk away more easily, causing patchiness on cladding and early track marks in decking.
Penetration is dependent on how the cell structure of the timber is formed and its condition e.g. broken open or still closed off.
In addition, stains can only penetrate while they are in a “liquid” form, thus for fast drying modern water borne stains this does not allow much time for penetration before the stain dries and changes from a liquid to a solid.
Penetrating oil stains such as Protector Water Borne struggle to penetrate into smooth dressed timber, as the wood cells have effectively been crushed by the planer knives. Slow drying oils like this can become subject to ‘over loading’ of coating on the wood surface leading to mottled effect.
Smooth dressed timber must be thoroughly sanded prior to coating with 120grit sandpaper to open the wood cells. Decking timber should be left to weather a few weeks and given a wash down with an oxalic cleaner such as Abodo Rejuvenator prior to coating.
Hand application of penetrating oils should be made sparingly and excess buffed back with a cloth after approx 30-40mins drying.
For interior applications, specialty ‘high solids’ oils or polyurethane coatings should be considered due to higher expectation on finish.
Earlywood vs Latewood
In addition there is a difference in absorption of stain into Earlywood and Latewood sections of the grain, with less penetration in the Latewood, having a denser and less open cell structure. With Flat Sawn boards this is much more pronounced compared to that of Quarter Sawn boards. This is very similar to the Dress Faced board issue as the stain erodes from the harder more dense Latewood band areas more so than the more stain rich and open Earlywood grain bands.
Band Sawn or brushed are the preferred finish options for exterior timbers.
These finishes have a more open cell structure and, due to a larger effective surface area, absorbs a higher amount of stain both into and onto the surface. This provides more coating for the natural erosion process and so results in a longer re-coat period. These finishes also dissipate surface tension making coating application more forgiving.
Abodo recommends Band Sawn or brushed finish for exterior timbers.