Why didn't you varnish them? It is one of the questions received after the publication of the article about refinishing of the solid oak doors of the church of Pitar Moș in Bucharest. Re-reading the article, I realised that, although it was obvious to me why I had chosen Kreidezeit oil-based varnish, for someone who was a DIY enthusiast and wanted to repaint doors or windows themselves, the reasons for the choice were not very clear. I then replied to the person concerned by mentioning that the question deserved an article where the choice was explained point by point. Below is the promised article.
Sculpture and oil used in abundance year after year made stripping very difficult
The church doors are carved and have been covered for years with excess linseed oil, without the unabsorbed oil from the wood being wiped away. The biggest problem was the oil left in the carving had turned into a gummy substance and set the dust, spoiling the appearance of the doors. This was also the reason for refinishing them, so the first thing to be done was to remove the dirt.
A little aside for those of you who still don't know about linseed oil. It is a very good water repellent material but does not dry out in contact with air. Its drying is improved by boiling and adding siccatives (drying chemicals), but if a thick film remains on the wood, it still does not dry but turns into a gummy substance. That's why, after it's over-applied and the wood is allowed to soak in, the surface needs to be wiped clean.
Even if the stripping was done repeatedly and then the surface was washed with thinner, it was practically impossible to remove all traces of oil from the carving. The wood did not remain clean and in this case, the application of a water-based exterior varnish, the most resistant option, was excluded because it would have no adhesion and in a short time it would have peeled off, compromising protection.
The option of using an alkyd varnish, compatible with linseed oil, did not seem a good one to me because of the low resistance over time. Such products are used less and less, water-based varnishes being much more resistant. In addition, unlike oils, alkyd varnish makes a film that in type cracks and flakes off. Repainting is very difficult because all varnish must be removed before applying any topcoat, including alkyd varnish.
Kreidezeit oil-based lye I found it to be the best outdoor oil because it has linseed oil and tung oil which makes the wood very well protected. In addition, these are products we have worked with before, we know them and their resistance over time was already tested.
Water-based systems, although very durable, do not adhere to oily surfaces
Because water-based varnishes for outdoor use are the most recommended, and indeed the most resistant at present, I come back with a few clarifications. First of all, they are not to be applied to oiled surfaces. Expression "they get along like water and oil" is also valid here, i.e. the two materials do not get along, they are not compatible. If water-based varnishes are applied shortly after the surfaces have been oiled, the varnish slides off the wood and does not film.
If some time has passed and the oil on the surface has been washed away by the rains, it is possible that the water-based varnish will adhere to the surface, but the resistance over time will be short-lived. The remaining oil inside the wood expands during warm periods and pushes into the film on top, pulling it away from the wood. In this case it is recommended that the wood be rough sanded first (60 or 80 grit), wiped with a thinner cloth and then sanded again (150-180 grit) to smooth the surface. A water-based varnish can then be safely applied.
For a water-based system to have good resistance over time (8-12 years) it must be professionally applied and applied according to the manufacturer's recommended technology. This implies spray application, as controlled an environment as possible, no extreme temperatures, sanding between coats, etc. When it comes to DIY projects, these requirements can rarely be met, which is why the durability is lower.
A clarification on the durability mentioned by the manufacturers. The 8-12 years refers to when the first cracks appear in the film, not when it is flaking or peeling. Checking surfaces and maintaining them every 4-5 years prolongs the life of the finish by taking the stress out of refinishing.
Yacht lakes, praised for their strength, are not always recommended
Another question when discussing the reasons for using oil and not varnish was why we didn't use varnish for yachts, as they are known to be very durable. Even though they are specifically formulated to last in water, these varnishes are not recommended choices for protecting wood outdoors because conditions are different. For boats, especially those that are also used at sea, you need a product that can withstand water, high and constant humidity and high salinity conditions.
Outside are different conditions, with protection concentrated in those directions. Humidity is variable not permanent, and the wood is swells when humidity is high and shrinks when humidity is low. That's why it needs an elastic product, a property that boat varnish lacks. Oils are recommended precisely because the lack of film allows the wood to vary in size without the risk of cracking.
I hope you find the information useful. As always, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.
Tagsoil finishing
