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Other major causes of coating breakdown are the methods, techniques and conditions of application. In most circumstances, it is an exterior application where problems more commonly occur.
Coat Surfaces all Round
Any area which can potentially allow moisture ingress should be adequately coated to form a "skin or "envelope". It is no use treating 99% of the substrate if water can still access the remainder. Tops and bottoms of doors are particularly at risk, as the door often hung before coating, and it these areas which tend to be splashed with rainwater. Care should also be taken to ensure that the weather bar is fully coated and sealed before fixing and the joint is then capped off with a suitable elastomeric sealant. Failure to treat the back of cladding can also give rise to problems with the coating.
Fully Treat End Grains
The end grain of the timber is many times more absorbent to water than the other faces. Often the ends of the cills (particularly above ground floor level) are neglected or receive cursory application, whereas they should actually receive more coating to ensure optimum performance.
Protect Against Moisture/Weather
The force that can be exerted as moisture penetrates the end grain and distorts the timber can be sufficient to force out nails and screws, resulting in permanent damage.
Apply Sufficient Coating
Sikkens exterior wood stains are gradually eroded away by the weather which is what makes the coatings so easy to maintain. However, if insufficient product is applied initially the coating will become too thin, sooner than expected. It is therefore imperative that enough coating is applied. This is typically found on a window which was ”decorated" but not protected by a sufficient film thickness. The most exposed surfaces have eroded prematurely while verticals are still in a good condition. Thin coatings will also be relatively easily stretched beyond their elastic limit. This results in a break in the coating eventually leading to the possibility of a complete breakdown of the finish. Brush application rarely achieves a wet film thickness of more than 60 micrometers for stains and 100 micrometers for opaques. This is less than one-tenth of a millimetre, so it is impossible to gauge or measure with conventional means. It is for this purpose that a "Wet Film Thickness" gauge is used. A Wet Film Thickness tool is a special gauge, they can be got from professional suppliers The teeth are cut at different heights above the level plane of the two outer teeth. This degree of accuracy is quite adequate to assess the thickness of the coating.
Stir Product Thoroughly
Stir the product thoroughly before, and regularly during, application, as constituents such as matting agents can settle out, and others such as pigments may either settle or float. If these are not kept evenly distributed using a broad bladed stirrer which reaches the bottom of the container, the finish can vary in colour and sheen from beginning to end of the work. This is especially critical with low build, low viscosity products, which do not have sufficient "body" to retain the solids in suspension.
Waterborne or Solvent-borne Systems
Be consistent with the use of either a complete waterborne or complete solvent-borne system to prevent residual solvent affecting the adhesion of subsequently applied coatings. Provided sufficient time is left between coats, however, there is no real problem of incompatibility. Three to five days in between coats is recommended instead of the usual drying times if applying a waterborne system over a solvent-borne one.
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Brushes
Good quality, long haired bristle brushes are always recommended except in the case of waterborne coatings when synthetic bristle brushes must be used. This is because natural bristles absorb water becoming soft and misshapen and do not allow an even application of waterborne products, leading to brush marks and an inadequate film build. Modern good quality synthetic brushes can successfully be used with both water- and solvent-borne coatings.
Drying Times
Just because a coating feels touch dry, it is not necessarily dry enough to overcoat. Waterborne coatings must lose organic solvent as well as water, and solvent-borne coatings must "cure", a process which usually takes at least 12-18 hours. Bear in mind also that adverse drying conditions (e.g. cold, damp, little ventilation) will extend (and can even double) drying times. While waterborne coatings can often be fully dry in less than 4 hours, solvent-borne coatings can take up to 24 hours, even in good drying conditions.
Patch Prime / Bring Forward
When applying maintenance coats, bare timber surfaces should receive an appropriate first coat, whether it is a base stain or a primer. In the case of opaques, the reasons are for adhesion, opacity or as a blocking agent, or a combination of all three. For wood stains, as well as the base stain, it is often necessary to "bring forward the base stained section with sufficient coats to restore the same shade as the surroundings. If this is not done effectively, then patched areas will stand out from the remainder of the substrate. The only alternative is to strip the whole section back to bare timber and re-coat.
Weather Conditions
While it is common sense NOT to apply coatings in the rain, if the coating is not dry before rainfall, then the finish can be damaged, and at worse washed off. Furthermore, when rain is imminent, the high humidity may also create drying problems. In terms of temperature, as a general rule of thumb, waterborne and solvent borne coatings should not be applied or allowed to dry outside the range of 8°C to 25°C and 5°C to 30°C respectively.
Treatment Sequence
The general treatment sequence is as follows:
- 1. Surface Preparation
- 2. Preservative Pre treatment
- 3. Base stain
- 4. Finishing Coats
- 5. Maintenance Sequence
If the original wood stain has been correctly applied then the only maintenance steps required are those in black. However, if this is not the case, or if the surfaces have been left too long before being maintained, additional steps shown in red are necessary. - Remove loose and flaking coating
- Remove and treat algae/mould
- Sand to remove weathered timber
- Clean other sound surfaces
- Remove resin and degrease
- Patch prime with base stain
- Bring forward to even colour
- Apply one or two coats of the appropriate Sikkens wood stain.
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