Could you paint over the sealing coat?

Paint over varnish or shellac can be applied in the same way as paint over seal coat, but you’ll need to use a high-speed sprayer for that application.

To paint or not to paint, that is the question

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

What to Look Out for When Painting Over Sealants

Construction projects often include interior painting which can result in the desire to paint over sealant. This is done so the painting and decorating trades don’t have to mask off the sealant. Reasons for painting over sealants include time savings, color matching sealants to compliment the proposed interior finished, avoiding the challenges of custom colors, eliminating returned or extra sealant, “paint out,” obscuring, or hiding sealants and sealed joints from view, and finally to allow for future refreshing of interior finished and design without cutting out and re-caulking.

Sometimes though, coating over sealants can be problematic. There are several unwanted results that can occur as a result when the sealant chemistry and modulus are not taken into consideration. To help identify and avoid these potential pitfalls, we have complied a list of four common areas to take into account when preparing to paint over sealants.

1. Painting Film Cracking
   1. Sealants are designed to be elastic and absorb movement, with the purpose of sealing joints where some degree of movement is expected. Provided the movement is within reasonable limits or within the capability of the sealant selected, the sealant will offer the satisfaction desired. Most all paints are designed to be hard, resistant to abrasion, and adhere to substrates, which do not encounter movement. If movement should occur in a joint sealant with a caulking, which has been painted, the caulking will move, and the paint will crack. If this is unacceptable to the owner, these moving joints should not be painted.
   2. In addition to movement, paining a sealant which has not fully cured may also cause cracking. Depending upon the exact type of sealant, the cracking can be caused by solvent evaporation or sealant shrinkage during cure. Because of this, painting should always be delayed until the sealant is completed cured.
   3. Incompatibility of the sealant and the coating can also come into play with regard to a differential in surface tension. This would be similar to water beading up on a freshly waxed surface. There are instances when the combination of sealant and coating can produce this effect which usually results in a network of fine cracks on the coating surface where it comes into contact with the sealant. A bridging primer can sometimes solve this problem with the selection of a primer based on sealant and coating chemistries being utilized.
   4. The cracking of rigid paints while not visually appealing does not constitute a failure of the sealant/paint system. The paint may be well adhered to the sealant and will only exhibit cracks while the joints is open in cold weather. Assuming the sealant was applied during warmer temperatures while the joint is closed.
   1. Paint Film Not Adhering to Sealant
      1. Some combinations of paint and sealant will result in the paint not adhering to the sealant, which will eventually result in the paint peeling away from the sealant. Testing of the paint and sealant combination is the only way to avoid such failures. These tests can be conducted in the field or in the lab. One simple test is to apply paint to fully cured sealant, allow to dry, and then use the “crosshatch” test as follows:
         1. Use a razor knife to score the paint film in a crosshatch pattern creating 1/8” blocks for about a one square inch area.
         2. After scoring apply duct tape firmly to the crosshatched pattern.
         3. Pull the duct tape from the cross hatch.
         4. Inspect the cross hatch for missing blocks.
         5. If 90% of the crosshatch pattern remains intact, the test would be considered passing.
         6. For further confidence, one may probe the crosshatch pattern with a razor knife trying to pull away the paint chips. They should be difficult to move.
         1. Paint Film Discoloring
            1. It is difficult to accurately predict whether or not a stain or discoloration of a paint film may develop when applied over a sealant. Both sealant and paint manufacturers employ a variety of raw materials which may not be completely compatible when exposed for prolonged periods of time. Most manufacturers will conduct compatibility testing using ASTM C-1087 as a model to ensure compatibility of materials.
            1. Paint Film Will Not Dry
               1. Some paints when applied to one component urethane sealants will not sure or dry properly. Paints, which exhibit this behavior, are usually alkyed-based materials with drying additives that get absorbed or deactivated by the sealant material thereby causing incomplete drying. A simple field test to verify proper drying would aid in avoiding this condition.

               Although some of the issues associated with painting over sealants may seem daunting or problematic, don’t fear. Field mock-ups along with laboratory testing in co-operation with the paint or sealant manufacturer will ensure good end results. For more information or testing questions, we urge you to contact our Pecora Technical Service Department. Our expert team is highly qualified to answer any questions, recommend the proper solution, or provide testing training. Our Technical Services staff offers training in product technology and use and provides the technical assistance you require in the planning and implementation of your project. They can be reached by email at [email protected] or by phone (800) 523-6688.

               
               

               Is Seal Coating Asphalt Actually Effective?

               There is no guarantee that seal coating over asphalt will be effective, but you may be able to improve the surface finish if you apply seal coat over asphalt.

               Seal coat over asphalt may provide some protection against weathering and groundwater seepage, but it’s not guaranteed to be an effective solution for all types of pavement.

               Can you put topcoat over shellac?

               Yes, you can apply a topcoat over shellac. If you apply a latex paint finish coat over the seal coat and want a smooth finish, you’ll need to sand the latex paint finish coat down before applying the clear coat.

               You can paint over shellac as soon as you apply seal coat over asphalt but you may need to sand latex paint finish coat down before applying the clearcoat.

               The sealer will stain the latex paint finish coat, so you’ll want to apply the latex paint finish coat over varnish or shellac first.

               How long does it take for shellac and sealer layers?

               It’s important that you don’t wait for more than a few hours between seal coat over asphalt and latex paint finish coat over seal coat.

               You may apply a latex paint finish coat over shellac in the same way you apply a latex paint finish coat over seal coat, but you’ll need to sand down the latex paint finish coat before applying a clear coat.