Timber Preparation: Wood is an excellent building material and used for various interior or exterior projects. Its versatility comes with a unique set of challenges when coating the substrate. Preparing natural wood for painting or staining correctly is a must to ensure a beautiful, long-lasting finish. Knots, moisture & sap bleeding will affect the final coating if not prepared properly.
Once we start painting any substrate, we must always begin with a clean, dirt and grease-free surface. If there is mould, dust, grease or anything else present, we must remove and clean it before applying any coatings.
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When starting to coat natural raw timber, first things first, we should check its moisture content. Painting natural timber with high moisture levels will result in product failure a loss of adherence. If you do not have one already, a moisture meter is an excellent investment to make, especially for painters and decorators. It can cost you more, in the end, with maybe having to return the following year and deal with a coating issue. Typically, exterior wood surfaces should not have more than 15-20% moisture present before painting & 12-15% for interior surfaces.
Priming Natural Wood
If you are deciding to paint bare natural wood, you should seal all the knots (spot prime) with a shellac-based or oil-based primer, or even a water-based exterior wood primer; the latter does raise the grain of the wood slightly, so you will have to lightly re-sand before a final coat is applied, always sand between each coat you apply, then dust off for the best results. Shellac and oil-based primers are great for sealing wood knots and sap streaks.
However, they carry a pungent odour (whereas the water-based primers do not) but tend to dry very quickly and will not let the knot bleed through your top (finish) coat. If you are looking to stain your natural wood, you don't have to seal the knots with a primer; slowly but thoroughly wetting the knots with stain will do. Remove all dried up and oozing sap is a must, though (NOT with white spirit or turps), especially when dealing with woods such as fir or pine. After removing the sap build-up, use an appropriate primer/sealer to seal in the sap affected areas.
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Filling in the Holes
And finally, it is time to fill in the holes and cracks and begin sanding. Use an interior wood filler (if inside only) or exterior grade epoxy filler for outside wood when filling in gaps and splits. I would recommend using a higher grade epoxy wood filler that shrinks or cracks less, like Dry Flex 4 2-in 1. Also, when it comes to sanding, I would stick with 150g – 220g sandpaper. And make sure that all your sanding is in the direction of the wood grain. If you Sand across the wood grain will create scratches that will absorb stain and polyurethane unevenly. When you're preparing your natural wood surface correctly, it will allow your coating to penetrate and adhere properly. Your new bare wood surface will last for many years to come.