To paint or not to paint, that is the question

In today’s construction, different types of sealants are used: silicones, acrylics, hybrids and PU sealants. If the aim is to overpaint the sealant after application, it is worth reading further because not every product is suitable for this purpose. 

In the case of silicones, it is absolutely clear – they are not paintable. The reason is that, after curing, the surface of acetoxy and neutral silicones is very smooth and their surface energy is low, so conventional paints do not adhere to them. Therefore, a product with a suitable colour shade must be chosen upon purchase, especially if you intend to use it in areas where the silicone remains visible after installation, for example, for bathroom joints.

In finishing jobs acrylic sealants are typically used. These are elasto-plastic, water-based products that are commonly used for joints with little or moderate deformation. The application of acrylic is easy and convenient, and after curing it can be easily painted with different types of paint, such as acrylic and alkyd paints. During the curing process of an acrylic sealant, there is always some drying shrinkage due to the release of water. Therefore, in order to obtain the best end result, the product must be completely cured before painting. Otherwise, the paint coating may crack on the surface of the acrylic. The final curing of the acrylic depends on the size of the joint, the ambient temperature and the humidity level. Whenever possible, painting should be carried out on the following day or at least 4-6 hours after application of the acrylic. A lot depends on the quality of the paint. To speed up the painting process, more elastic paints should be preferred.

As a result of the development of acrylic products, so-called siliconized acrylics are increasingly used. Their mechanical properties, such as elasticity and mobility, are quite close to silicones. Therefore, siliconized acrylics are also called paintable silicones or painters’ silicones. By their very nature, however, they are a water-based product, which cures in the same way as ordinary acrylics and is therefore also paintable.

Hybrid and polyurethane sealants are similar in terms of paintability – both can be painted with water-based paints. The advantage of these sealants compared to acrylics is that there is virtually no drying shrinkage during curing. Thus, both hybrid and polyurethane sealants can be painted almost immediately after installation. An important aspect that should not be forgotten is that given their properties, hybrid and polyurethane sealants are often used for joints with major deformations. For example, today there is a normal expectation that a façade sealant must be able to compensate for deformation by ± 25%. When finishing the joint sealant in such place with a low-elasticity acrylic paint, the paint coating will start to crack on the sealant surface sooner or later. In order to avoid this problem, a hybrid or polyurethane sealant that does not need to be overpainted should be selected for such joints.

What should be taken into account when painting the sealants?

1. Always test the compatibility of the sealant and paint before performing  the work. Today there are a large number of different products available, and new products are constantly being developed.
2. For best results, allow the sealant to fully cure before painting.
3. Follow the instructions and restrictions of the manufacturer of the paint  and sealant as set out in the product label and in the technical documentation.

 

Text: Kuldar Kongo, Product Manager

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2020