Let’s critique some of the finishes available on the market today:
- Product failures due to lack of UV protection.
- Hard shell finish, no flexibility.
- Non-micro porous, which does not allow substrate to breath, so moisture behind the polyurethane causes it to blister and finally flake off.
- Requires rework and major restoration after 12-18 months.
We would not recommend urethanes for exterior use, without the addition of UV absorbing chemicals at prohibitively expensive concentrations.
- Product failures due to oils inclination to migrate under head back to the surface of the timber and weather away.
- Some unmodified oils are actually a good source for mould and lichen.
- Generally require recoating approximately every 7-10 months.
The use of vegetables oils e.g. linseed oil for finishing timber entrance doors is not recommended.
Penetrating stain or low build semi-transport finishes
- Product failure due to weather by erosion – once this has occurred water resistance is lost leading to moisture uptake of timber results in cracking and splitting.
- Renowned for high instances of mould growth.
- Generally require recoating approximately every 12-18 months.
These finishes form a very thin film on the surface of the timber and can only be regarded as temporary wood protection agents.
Film Forming Transparent Finishes:
- No product breakdown or discolouration (silvering) due to high UV protection.
- Surface showing no signs of cracking or flaking (due to flexibility of coating) micro porous coating.
- Water repellence is excellent.
- High quality look due to excellent film build
- Generally re top-coated after approximately 3 years in high sun areas
Recently developed alkyl resins incorporate a greater flexibility and UV light resistance than earlier products and promising results have been obtained from exterior weathering trials.
All of the above are the result of trials taken by BRANZ in November 1998 and confirms that for exterior entrance doors a firm foaming product is by far the best to use.
Let’s look at the characteristics used in a solid timber door.
Timber is made up of a combination of porous cells which when dry (ideally 12-14% for the manufacture of doors) the cells become hollow. This leaves room for moisture to absorb into the timber and if not sealed properly problems will eventually arise. As the timber dries out moisture will penetrate causing the timber to move thus bowing or causing twist in the door or even splitting.
All oils and penetrating stains are no sealers! They line the inside of a cell and form a thin coat over the face until the heat from the sun draws the strain to the surface of the timber allowing moisture to penetrate. Then the problems start…
As stated above, we recommend film-forming products. Examples of brands are Sikkens, Intergrain & Cabots. An easy option is the DIY product – one includes all the needed products and equipment in one simple kit.
These brand of sealers cover and protect the timber far superior to other forms of finishing and will not allow moisture or the outdoor elements to penetrate through. This keeps problems, like bowing, cracks and splitting, at bay.
This product is applied to each door consisting of three coats. The base coat being a transparent primer – is water repellent and protects timber from effects of UV exposure. The top coats (stain sheen finish) give a flexible coating, which allows the door to give natural movement without breaking the seal.