Why water-based varnish or paint does not work over linseed oil

Someone asked me if they could apply a water-based varnish that is resistant to the outside over wood treated with linseed oil. The varnish was recommended by a friend who had used it and was very pleased with the result. His friend had applied it directly to the wood, but the questioner had heard that it was best to treat the wood with linseed oil first. My answer was: no. Water-based varnish over linseed oil does not work because water and oil simply do not get along, they repel each other! It doesn't work even if it's been 3-4 days after applying the oil, as I saw a recommendation on a profile group 😐 Even if at the moment everything seems fine and nothing happens, serious problems can occur over time. I'll explain why in a moment.

Water-based varnish does not work over linseed oil even 3-4 days after application

Linseed oil applied is absorbed by the wood inside, it does not remain on the surface. In fact, it doesn't have to stay because it doesn't dry in contact with air and doesn't form a film. This is why a surface over-treated with linseed oil and not dried becomes sticky. Usually 2-3 coats of linseed oil are applied, with a "drying" time of 24 hours between coats. This is actually the time allowed for the wood to absorb as much oil as possible into the interior.

Because it does not film, 3-4 days after application, especially in cases where the amount applied was not very large and the wood is not saturated, the oily appearance of the surface may disappear. A water-based product applied to this surface no longer slips very obviously and does not form the characteristic islands. If the sanding has been coarser, the varnish is still absorbed by some of the open wood fibres and this creates the impression that the water-based varnish has worked very well over the linseed oil.

The problem can occur during very hot summer periods. At high temperatures, the oil inside expands and pushes into the varnish film which doesn't have a very good grip on the wooden surface anyway. This causes the film to peel off from place to place, enough to allow water to get in. The varnish begins to peel and in some places the wood begins to blacken due to the appearance of mould.

A water-based varnish or paint can be applied to an oil-treated surface months after application, when the linseed oil has been washed off by the rains and the wood has taken on a dry appearance. The presence of oil in the wood gives it a slightly damp appearance. Perhaps for this reason the oil is said to nourish the wood. When the wood looks dry, lifeless, without the silky sheen of oil, a water-based product can be applied to the wood.

So what product goes over flax oil?

Alkyd or oil-based products. I.e. varnishes or paints that use white spirit (petrosin, gas, petroleum-derived solvent) or turpentine (thinner extracted from resinous wood) for dilution. These are compatible with oil and do not slip on surfaces treated with linseed oil. But beware, they will not adhere well to sticky oil surfaces. Before application the surface should be wiped with white spirit to remove the gum formed by the excess oil.

Linseed oil is a good protection against moisture, but works best in combination with tung oil. Danish oil or Kreidezeit oils are such examples of blends of linseed oil and tung oil with very good results in wood protection. The molecule of linseed oil is smaller than that of other oils and penetrates faster and deeper into the wood. But the fact that it doesn't make a film allows water to remove it more easily, and a molecule that goes in easily can come out just as easily when the surface is subjected to the action of flowing water (rain, shower areas, etc). Plain linseed oil is one of the least resistant solutions for outdoor wood, with no protection against UV rays, protection only given by the addition of pigments.

To ensure long-lasting durability, use protection systems that are tested and guaranteed by the manufacturer.

If you want to protect outdoor wood, choose systems that are tested and guaranteed by the manufacturer. Whether oils, solvent-based products or water-based, choose all the system from the same manufacturer and follow the recommended technology to ensure long-lasting durability. Protect the wood also on the parts that are not visible because water gets into the wood there and reduces the resistance of the finish. If you are doing an exterior job, finish the components before fitting to ensure you have done the wood protection right. At the installation site, finish only the wood left unprotected after adjustments.

To increase the strength of exterior finishes over time, have them regularly maintained. Oils don't suddenly disappear from wood, and varnishes don't crack and flake instantly. If you apply another coat of exterior oil on top of the existing one before you get back to the wood, you will have a well-protected fence or deck again. And applying protective products on the lake film prevents small cracks from appearing in the film so that water can no longer enter and the lake does not run off.

I hope you find the information useful. If you have any additions or questions, please leave them in the space below. I will certainly reply.