Preparing Bare Wood for Painting

 

There are hundreds of species of wood used to build a vast array of articles and structures for both exterior and interior exposures. Wood is also unique because it is the one substrate that can have an opaque pigmented finish; stained pigmented finish; or a clear finish.

Surface Preparation, General
Regardless of whether a pigmented, stained or clear finish is specified, the following are recommended practices for proper wood surface preparation.

Wood can be clear coated to highlight it's natural finish.

 Determine substrate
Before you carry out any cleaning it is best to determine what type of coating has been previously applied to the surface or substrate (if any)

Low gloss coatings -  are less durable than higher gloss coatings making them more susceptible to damage from harsh abrasion and chemical cleaning.

High gloss coatings - are more durable, so will endure more abrasion and chemical cleaning, but are less often used on interior walls because they tend to enhance any surface defects, making them more visible.

 Measure for moisture. Moisture readings should be taken, Exterior surfaces can not have more than 15% moisture present prior to painting; interior surfaces, 12%.

Remove surface contaminants. All grease, oil, dirt, dust, or other foreign matter must be removed from the surface. Avoid using steel wool to clean the surface: small fragments of steel wool can get caught in the grain and cause black spots to form long after the wood has been finished and installed. Rough surfaced woods can be cleaned of most loose material with a wire brush. A stiff fibre or hog’s bristle brush is recommended for smooth finished woods to reduce scratching.

Remove sap and pitch. Especially on softwoods such as pine and fir, all sap and pitch must be removed. All pitch pockets and sap-effected areas must be sealed with an appropriate sealer. If a stain or varnish/stain system is to be applied, the spot sealer must be applied after the stain and allowed to fully dry, otherwise, the colour may appear blotchy and inconsistent.

Deal with knots: If a paint system is specified, all tight knots should be sealed with an appropriate knot sealer. If a semi-transparent stain system is specified, knots should be left unsealed or the colour of the finish may appear patchy.Loose knots should be removed and filled with an appropriate caulking compound.

 Fill the holes: Holes, cracks, and joints are to be filled with an appropriate filling compound level with the surrounding surfaces. Openings in excess of 1/8"inch (3.18mm) should not be the responsibility of the painting contractor.

Sand with 150 grit sandpaper: For finished lumber, sand out any raised grain with 150 grit sandpaper before and between coats to maintain a uniform appearance. All sanding must be done in the direction of the grain, particularly when a stain or varnish system is specified. Sanding across the grain creates scratches that will absorb stain or varnish unevenly, a flaw that will need to be corrected prior to coating application; we’ll discuss this in more detail below. Dust particles from sanding should be removed by wiping the sanded surface with a tack rag, a loosely woven cloth dipped into a varnish oil and wrung out to make it tacky or sticky.