What is Factory Primed Timber?
Timber that has been primed by the manufacturer on a factory production line, using high quality industrial primers such as waterbased acrylic or solvent based enamel primers. Factory Primed Timber is produced using engineered timber such as MDF and “Weathertex®”, and pre-treated plantation softwoods such as Radiata Pine or Hoop Pine.
Organic Fibre-Based Building Boards sometimes called insulating boards, wallboard, particleboard, chipboard, and hardboard are often supplied factory pre-primed ready for painting. Bare edges or cuts need to be well sealed prior to painting.
Factory Primed Timbers for external use should not be allowed to weather for more than 1 month before top-coating, as any surface contamination, chalking or significant water uptake may hinder the adhesion or fundamental performance of the subsequent paint coating system.
Why is Factory Primed Timber used?
Factory Primed Timber is now widely used throughout the building industry in place of the higher quality and more naturally durable timbers that have become harder to obtain and have increased significantly in cost over time. Pre-priming of timber on a factory production line is a very efficient means of producing high quality primed timber board, that is ready for site installation and top-coating.
However, not all pre-primed timber is suitable for external use and only high quality Factory Primed Timber eg “Weathertex®” should be used for exterior applications. These pre-primers should not be confused with premium quality architectural primers which are crucial to the durability of a complete paint coating system.
Unfortunately low quality post-primed timber is often mistaken for high quality Factory Primed Timber ready for topcoating, sometimes because they look the same colour.
Some exterior grade Factory Primed timbers are branded to identify the manufacturer and are factory-coated with a higher quality architectural primer ready for on-site painting. In this case the manufacturer would generally provide the recommended over-painting guidelines. If a good quality Factory Primer has in fact been employed, then it should be quite suitable for undercoating with a high quality exterior grade prepcoat or even topcoating.
On the other hand, some non-branded timbers are simply pre-primed to temporarily protect against degradation from weather and UV attack, as well as soiling during construction. Unfortunately, the quality of these pre-primers can be very unreliable, as they are likely to be low in resin or binder, high in solvent and pigments, resulting in a poorly bound paint film.
Solutions for painting Factory Primed Timber
Where Factory Primed Timber is to be used, the following recommendations need to be put into place.
Factory Primed Timber
Not all pre-primed timber is coated with high quality factory applied primers.
Poorly bound pre-primers used on some grades of timber can be recognized by simple testing of the mechanical integrity of the coating :
- Soft film easily scratched with thumbnail or coin
- Very dull or flat sheen level compared to a conventional timber primer
- Less than half (typically around 10μm) the film thickness of a conventional timber primer
- Porous and easily sanded without any clogging of the sandpaper
- Poorly adherent and may be easily removed with clear adhesive tape
Due to the number of variables and unknowns associated with pre-primed timber, it will be necessary to conduct a series of crosscut adhesion tests to verify the film integrity of the pre-primed surface. (Ref: AS 1580.408.4 – Crosscut Adhesion Testing of Paints).
Preparation of “unsound” pre-primed surfaces for Painting
If the pre-primed surface is powdery, flaking, sticky or if any of the existing pre-primer comes off during the adhesion testing, then the surface is considered “unsound” and NOT suitable for further painting. The unsound pre-primer must now be completely removed by sanding and/or stripping.
Given that an unsound pre-primed surface is a poorly bound thin porous paint layer, sanding it off completely will require surprisingly little time or effort. Once the pre-primer is removed, the timber should then be tested for water repellency by spraying or sprinkling fresh clean water onto the surface.
The water should NOT form “beads” on the surface but should actually be absorbed into the timber.
If the water “beads” it indicates that the surface is “water repellent” and will need to be washed thoroughly with a solvent (eg, Mineral Turpentine) to remove excess waxes or oils that may be present.
When the prepared and cleaned timber surface is ready for painting, then the Prepcoat/Primer can be applied directly to the prepared timber surface (see Prepcoats/Primers…below).
Preparation of “sound” pre-primed surfaces for Painting
If the pre-primed timber passes the adhesion test, then it is considered “sound” and the Topcoat can be either applied directly to the pre-primed surface; or a Prepcoat/Primer can be applied to the pre-primed surface for additional performance, prior to topcoating (see Prepcoats/Primers… below).
If the timber is tannin rich apply a high quality “Tannin Stain Blocking” oil-based primer such as Dulux® Precision High Opacity Stain Blocker. Allow to dry adequately prior to topcoating with water-based Acrylic paint.
Where tannin is not a problem, apply a high quality, tannin resistant 100% Acrylic Primer to the prepcoated/pre-primed surface prior to topcoating with water-based Acrylic paint, followed by two coats (minimum) of an exterior grade 100% Acrylic topcoat.
In order to inhibit the growth of mould or mildew on Factory Primed timber and reduce the long term effects of weathering, it is essential to employ a good quality, UV resistant, 100% Acrylic topcoat, paint coating system on all pre-primed exterior timber surfaces without delay.
Generally speaking, two coats (minimum) of an exterior grade 100% Acrylic topcoat, such as Dulux® Weathershield or Dulux® Aquanamel will provide the best long term performance in all environments.
Where Timber Stains and/or Clear Finishes are to be employed instead of paint coatings, contact Cabot’s® Customer Service for technical advice.
What steps can be taken to avoid problems?
Factory Primed Timber surfaces should be below 15% moisture content before painting.
Exposure of Factory Primed Timber surfaces to the weather for long periods (generally more than 1 month) should be avoided as it will dramatically reduce the integrity of the primer and compromise the adhesion of subsequent paint coatings.
For best results, high quality Factory Primed Timber should be painted within one month of being exposed to the elements.
Further information on the preparation of unpainted timber and the top-coating of Factory Primed Timber can be obtained by contacting Dulux Customer Service.
Further information on the preparation and painting of Organic Fibre-Based Building Boards can be found in AS/NZS 2311 – “Guide to the painting of buildings”. Section 3.3
Related technical advice
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