A beautiful and organic material, professional appliers know that wood can also react in an unpredictable way if treated incorrectly. Using the wrong product can result in costly delays and wasted man-hours (and lots of stripping and sanding!), so it's important to know how to deal with wood that's been previously coated as well as new or bare timber. After all, it's more likely clients will call on your expertise to deal with damaged wood rather than brand new timber that's easier to treat.
Here are the most common problems to arise with previously coated wood and how to combat them:
1. Peeling or flaking coating
The main cause of peeling or flaking coatings is lack of sufficient preparation before applying products. Silvery-grey timber fibres are a sign of timber that has been 'denatured' by weathering. These fibres are absorbent but loosely attached to the main body of the timber, so if they are not removed before coating, the stresses of external exposure will cause them to become detached from the surface, taking any coating with them. Therefore the these grey fibres as well as any old, flaking coating must be fully removed before treating the wood again, and in some cases it will need to be sanded too.
2. Uneven weathering (especially for woodstains)
If a woodstain has weathered unevenly, it's best to sand back the timber until the weathered areas have disappeared before applying a new woodstain or coating.
3. Exposed timber - denatured/weathered and grey
As mentioned above, silver-grey fibres indicate that wood has been exposed to UV light over time and has weathered. If timber has been exposed to direct sunlight for more than four weeks it will require mechanical sanding of the surface in preparation for a coating.
Wood is absorbent so naturally absorbs water, making it an ideal environment for mould or mildew to grow. If the wood you're dealing with has been painted or stained, the mould should not have penetrated through to the wood beneath the coating. In which case, general cleaning and a fungicidal wash such as Weathershield Multi-Surface Fungicidal Wash should be sufficient to remove mould or algae. It's worth investigating the source of the mould or damp as you would with walls - it will reappear if the root of the problem is not addressed. If the mould is evident beneath the coating, remove the coating by sanding or scraping, treat with Weathershield Multi-Surface Fungicidal Wash.
Your professional judgment is key here. If the wood can be salvaged, the first stage to dealing with decay is to cut out the affected area. If the wood is wet, allow it to dry before filling with a wood filler such as Cuprinol Ultra Tough Wood Filler. For best results, pre-treat the repair area with Cuprinol Clear wood preserver and allow it to dry. TIP: to obtain better anchorage of the wood filler in deep holes (greater than 25mm or 1 inch), fix screws into the bottom of the holes leaving the heads and stems exposed inside the holes, but of course, beneath the surface...
6. Knots bleeding/exuding resin
Resin exudation from timber is natural and highly unpredictable, and varies depending on both timber species and timber grade/quality. The traditional answer to apply shellac knotting to 'seal' in the resin, has been shown to be ineffective outside. While 'microporous' finishes such as woodstains allow the resin to filter through the finish without blistering or peeling, in severe cases this can still occur. Affected timber should be degreased before being coated using a sharp solvent such as methylated spirits. Any resin which exudes through the coating can be gently scraped off and wiped with meths, avoiding the need to redecorate any sooner than would be expected.
N.B. Degreasing with meths will also remove surface oils and gums from hardwoods, and greasy fingermarks from handling, so well worth doing even if you don't know the timber type you're dealing with.
7. Open joints and exposed end grain
End grain is many times more absorbent than the other timber faces, and it is important to protect joints by applying sufficient coating. In severe cases, it may be necessary to repair the joint using a suitable two-part filler, to prevent ingress to exposed end grains.
8. Iron staining
In the presence of moisture, standard steel fixings can react with soluble wood components and create a blue/black discolouration. This form of staining/discolouration is best treated with a wood bleach (active ingredient oxalic acid). Remove all surface coating prior to application.
Where iron staining has occurred, remove the cause of the discolouration - where this is not practical, any fixings should be driven home and the holes filled with a woodfiller. The problem is particularly significant with acidic timbers such as oak, Western red cedar and Idigbo, where non-ferrous fixings (e.g. stainless steel) should be used.
9. Holes (where filler is detached or missing)
Holes, voids or areas of damage in timber should be filled prior to coating, to help avoid moisture ingress and excessive timber movement. All purpose fillers are not flexible enough for use with wood. Only Woodfillers should be used.
10. Surface dirt/contamination - often overlooked
In virtually every maintenance job, dirt will have accumulated on the surfaces. Thorough cleaning of the surfaces is required to remove most of the contaminants which can impair the absorption, adhesion and subsequent performance of coating systems. A solution of household detergent in warm water and a stiff, non-metallic bristle brush is ideal for removing oil, dust, dirt or grease. Thoroughly rinse off any residues and allow the surface to dry fully.
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Watch our preparation videos on YouTube for more help on dealing with wood.