【Haichuan Anti-corrosion Knowledge】Why is a thinner added to paint? What exactly is the diluent used to dilute?

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The last edit to this post was made by HaiChuan LaoYu on 2026-4-27 at 19:00. When many people apply paint or coatings, their first reaction is to say, \"Add some thinner to thin it out, so it’s easier to apply and spray.\" ”But few people have seriously considered this question: what exactly is the diluent diluting? Many people’s intuition is that a thinner is something that “dilutes” paint. In fact, from the perspective of paint formulations, the thinner does not dilute the paint itself, but rather the resin component in the paint system – that is, the substance that forms the paint film. In short: the purpose of a thinner is to dissolve the resin system and reduce its viscosity, thereby making it easier to apply the paint. What exactly is in a bucket of paint? Further reading: How are the magical paints that extend the lifespan of steel created? A typical bucket of paint consists of roughly five components: 1. Resin (the film-forming substance), which is the most crucial component in paint. Determines properties such as adhesion, durability, hardness, and gloss. 2. Pigments provide color and coverage. 3. Filler: Increases volume, reduces costs, and improves certain properties at the same time. 4. Additives such as drying agents, anti-settling agents, leveling agents, anti-caking agents, etc. 5. Solvent: Paint already contains a certain amount of solvent upon leaving the factory; this solvent is used to control viscosity and ensure that the paint does not become too thick in the container. There is a point that can be confusing here: paint already contains solvents as part of its formula, while the \"thinner\" added during application is actually an additional solvent mixture. Why is a thinner still added during construction? Paint is usually formulated to be thicker (with higher viscosity) at the time of production. The reason is simple: it doesn’t settle easily during transportation, remains stable during storage, and doesn’t separate into layers. However, at the construction site, this viscosity becomes less suitable for application. For example: If the coating is too thick when brushed on, it won’t spread evenly; if it’s too thick for spraying, it won’t atomize properly. An overly thick coating also tends to sag. In such cases, a thinner must be added. Once the thinner enters the system, it dissolves the resin and reduces the overall viscosity, giving the paint the proper flowability to be applied more easily by brushing or spraying. In a nutshell: the function of a thinner is to reduce viscosity, making it easier to apply paint. What exactly are turpentine and nail polish remover (banana oil)? At many renovation sites or construction sites, the two names most often heard are: turpentine and linseed oil (banana oil). In fact, they are all organic solvent-based thinners in essence, with only different applications. Turpentine is also commonly referred to as: • Alcohol-acid thinner • Paint-thinning agent. Its main components are usually petroleum solvents (alkanes, cycloalkanes, and a small amount of aromatics). Features: • Strong odor • Moderate evaporation rate • Relatively low cost. Typical uses: • Alkyd paints • Blending paints • Enamel paints • Some anti-rust paints. Paints used for many old iron doors and windows, as well as wooden furniture, generally contain turpentine. Naphtha (banana oil). The composition of naphtha is usually more complex; common components include: • Acetates • Toluene • Xylene • Ketone solvents. Characteristics: • Faster evaporation • Greater solvency • Stronger odor. Typical uses: • Nitrocellulose paints (NC paints) • Wood finishing sprays • Automotive repair paints. Many people, upon smelling it, say, “This is naphtha (banana oil).” ” Using the wrong diluent – the most common “disasters” on site. On construction sites or in factories, using the wrong diluent is actually quite common. Many construction accidents arise from this. The common issues mainly fall into these categories: 1. Bottom biting, wrinkling, and cracking, which are the most frequent. For example: using thinner to dilute alkyd paint, or vice versa; when the solvency levels don’t match, the underlying paint gets damaged. On the surface, wrinkles, cracks, and orange-peel-like patterns will appear. The phrase most often heard on site is: “It was fine after being washed yesterday, but today it’s all wrinkled.” ”2. Orange peel effect, poor leveling – the evaporation rate of the thinner is not appropriate. Too fast: The paint starts to dry as soon as it’s applied to the surface; before it has a chance to level out, it already sets. As a result, orange peel texture, granular appearance, and a rough surface are formed. 3. Reduced adhesion and chipping: Incompatible solvents can cause resin precipitation, pigment sedimentation, and breakdown of the system. It’s not visible in the short term, but it may appear after a few months: bubbling, peeling, and complete paint loss. 4. Whitening and loss of luster: Some diluents may become water-containing or evaporate too quickly. During the film-forming process, the paint film may exhibit: whitening, loss of luster, and uneven color. It was originally a high-gloss topcoat, but after it dried it became matte; the customer was extremely upset upon seeing that. 5. Excessive odors and even safety hazards: If solvents are mixed indiscriminately on site, VOC levels may exceed the permissible limits significantly. Some cheap diluents even contain large amounts of aromatic solvents. Mild cases present with dizziness, nausea, and skin irritation. In severe cases: occupational health risks. One of the most important construction principles is that many experienced workers often say: “Use each type of paint and mixture separately; don’t mix them together randomly.” ”What does that mean? That is: every type of paint has a matching solvent system. It is best to follow the manufacturer’s recommendations: strictly control the type and ratio of the diluent. Typical dilution ratio: 10%–30%. It is advisable to conduct a sample test before starting the application to ensure there are no issues, before proceeding with large-scale application. Finally, I’d like to say that many people think: a thinner is nothing more than “adding something to the paint”. But in reality, behind paint application lies a whole set of chemical balances related to resins, solvents, and evaporation rates. A diluent may seem like just a “little bit” to add, but using it incorrectly once can result in having to repaint the entire surface, redoing the entire batch, or even causing safety accidents. Saving a few dollars on thinner often results in greater losses. If you’ve worked on construction sites and encountered cases where things went awry due to the wrong thinner being used, feel free to leave a comment and share your experiences, so we can all avoid such pitfalls.