Want a quality film? Choose the right thinner

I recently had a discussion about the importance of thinner in finishing. The person wanted to buy lacquer and primer, with the necessary catalysts, but without thinner. I explained that the thinner is just as important as the base products. If the wrong choice of thinner is made, the very properties for which he had decided to buy a quality product could be lost. Shortly after the incident a friend told me that he had exactly the same experience, followed by some complaints about the quality of the products. The incident convinced me that an article explaining the importance of thinner in the finishing process is needed.

Lacquers and paints are mixtures of resins, solvents and auxiliary materials. After application, the solvent evaporates, causing the film to dry. Even if the drying occurs as a result of a chemical reaction between two components, the diluent can be used to control the speed of the reaction and the time during which the mixture remains liquid (pot-life). In the case of waterborne products the solvent is water. Being a single component there is not much that can be done to influence the quality of the finish only by controlling the evaporation of water. In this case everything depends on the added auxiliaries, temperature and air circulation.

In contrast to water-based products, solvent-based products have diluents made from solvent mixtures. The proportion of them, their qualities, influence both the drying time and the quality of the film, and the proportions can vary according to the desired result. If you ask a varnish supplier for a thinner, you will instead get a question: which?/for what? Why does he need so much information when, if we really want to, with one thinner we can apply most products? Why have more than one thinner?

Because the application can be done, but that doesn't mean that just because it came out of the gun and sat on the object that the product will look good in the end. No one wants to complicate their life by keeping multiple products of the same kind in stock, because that means tied up stock and money. There is more than one because each has its role and only by using them correctly can quality finishes be achieved.

The simplest example is the existence of summer and winter thinners. In winter, when it is cold, a faster thinner is needed so as not to prolong the drying time. The same fast thinner will lead in summer, when the heat is high, to a gaseous film full of bubbles trapped in the film. In this case a heavier volatile thinner is needed and there are 2 possibilities. Buy a summer thinner from the start, or take a very heavy volatile thinner (retarder) and add 10-20% to the winter thinner to make it slower.

This way of influencing appearance is the most popular. However, you should be aware that thinners influence gloss, stain, appearance, film drying and can lead to finishing defects that are difficult to remove.

The influence on gloss is best seen with high gloss varnishes. If a quickly volatile thinner is used, the film does not have time to spread and the light reflection will not be perfect. A thinner can also decrease gloss by 20-25%.

The wrong choice of thinner can also result in a rough, gauzy film, as well as a soft film that looks undried or has poor scratch resistance. There is often a tendency to apply a thicker, undiluted or insufficiently thinned coat. The varnish does not spread well, leaving a thick layer from which the thinner, which has been in place since the varnish was formulated in the factory, does not have time to come out and gets trapped in the film. This results in a soft film, which scratches easily and can get imprints from packaging.

Much of the finish defects are primarily due to the wrong choice of thinner, or that too. Appearance orange peel, gassed film, the dregs, spots and color migration, lack of adhesion to the substrate, cracks in the lake, crater, gloss variation are the problems you run into if you don't choose the right thinner.

For these reasons I maintain that it's best to buy your thinner where you buy your varnishes. I know that a lower price is more tempting. Consider though that the price of the thinner is not that high, and 20-40% in the varnish does not result in a huge increase in cost/sqm. The cost difference is certainly much less than the cost of fixing any defects that may occur. Ask your refinish supplier for advice and choose the recommended thinner. In the end you will find it is the cheapest option.