When we refer to coatings that protect materials of construction – concrete & steel, for example – we often refer to multiple coats of paint. Combined, these paint coatings are known as a paint system. A coating system could be comprised of a primer, an intermediate, and a topcoat. Or, the coating could be made to be applied directly to the substrate in one coat. An alkali-resistant coating is an example of what may be used on a concrete water storage tank; a direct-to-metal coating is its example on steel tank surfaces.
No matter how you decide to protect your structure – what coating system you choose – the first coat applied is the most important. It’s the most important for one reason that is often overlooked: it must stick to the surface! If it doesn’t, the entire system will fail to perform in the manner expected. In previous postings, I discussed the importance of surface preparation and how to achieve it. This time, let’s discuss first coats or primers.
Remember that first coats / primers for steel may be very different from first coats / primers for concrete. A textured acrylic coating, used on a concrete water storage tank, would not prevent rust on a tank made of carbon steel. Similarly, a zinc-rich primer, providing some measure of galvanic protection on a steel tank, for example, would be wasted on a concrete tank.
Hydraulically placed cement (aka: gunite) used in the construction of concrete water tanks is alkaline. That is, its pH is initially high. To assure that the first coat / primer sticks to this alkaline surface, and provides a foundation for future coats (if specified), it must be alkali-resistant. Some acrylic coatings are … so choose those.
Zinc-rich primers, providing galvanic protection on carbon steel surfaces, have an additional benefit of resisting undercutting – often depicted as rust expanding under the coating system. Zinc-rich primers adhere, they provide some measure of galvanic protection, and they can reduce undercutting. For those reasons, consider selecting a zinc-rich primer as your first coat on carbon steel water tanks.
Want to know more about first coats / primers?