A primer is the first layer/coat applied to a substrate. It adheres to the substrate and provides the first layer of protection from corrosion, water, dust, UV radiation, and many other things. It also ensures adhesion to the subsequent coat of paint. 

Primers can primarily be classified into 5 categories depending on the substrate they need to adhere to. Below is a simple classification of the range of primers:

Primer for Iron

Iron is ductile, malleable, abundantly available, and the most economical metal. Hence many large structures including large ships are built with iron metal. However, iron is very easily susceptible to corrosion, also known as rust. This rust keeps on detaching from the parent iron metal and causes weakening of the structure. The function of the industrial primer paint is to protect the metal by inhibition effect, barrier effect, and /or cathodic protection.  Few examples of the application of primer to ferrous metal substrates include metal grills, ships, and prefabricated building structures. 

Primer for Non-Ferrous Metals

Nonferrous metals include alloys such as bronze, aluminum, gold, and many others. They form adherent protective oxide layers over the surface. Due to the nature of these materials, adhesion of paint and the properties required to protect these metals would call for different raw materials in the paint. Since these metals are less prone to corrosion, application including the ideal number of coats required, could also be less. 

Primer for Plastic

As per the morphology of plastic, it is one of the most difficult substrates to coat due to its low surface energy. The major role of a primer in plastic is to provide adhesion to the substrate as well as subsequent coat. The primer for plastic varies depending upon the type of plastic used such as PP, ABS, FRP, GRP, HIPS, SMC, etc. Accordingly the primer for plastic can be clear or pigmented. Similarly resin type can be CPO, Acrylic, PU, etc.

Primer for Concrete

Concrete is a mixture of various substances, namely, sand, cement, and water. The nature of this mixture makes the substrate alkaline and porous with a rough surface that is very hard. The primer should hence be alkali resistant. The primary role of a primer coat here is to fill the pores of the substrate, so it largely reduces the seepage and wetness in the wall, hence reducing the possibility of growth of algae and fungus. The secondary role is to provide a smooth, uniform surface for the topcoat. Hence, multiple coats (2-3) are applied after sanding each layer in its uneven areas to achieve a leveled surface. Topcoats applied on leveled surfaces tend to be visually more appealing because a leveled surface leads to a better finish.

Primer for Wood

Wood is a natural cellulosic material with beautiful naturally occurring patterns and figures, but also a porous surface. The porosity of wood varies based on the type of wood. In order to seal this porosity and achieve a smooth, uniform, and leveled surface, the surface of the wood is required to be coated with a primer. The choice of primer depends upon the final requirement of the user i.e. whether the user needs to retain the natural pattern and figure or not. If pattern and figure are to be retained then clear primer, which is generally referred to as a sealer is applied and for no pattern, an opaque primer is applied. Multiple coats are suggested to achieve the best result with scuffing between each coat to ensure the surface is leveled.

FAQs

Do I need a primer before painting?

Yes, primers are mandatory before painting for attaining the highest level of protection. It ensures adhesion to the substrate and to the subsequent coat. It also acts as the first line of defense against corrosion. Even during maintenance the application of primer is preferred on intact aged paint. 

What is industrial primer paint used for?

Industrial Primer paint is a primary layer of coating that is applied to the materials before the paint is applied. It ensures that the paint properly adheres to the surface and improves the adhesion of subsequent coat of paint. Primers in different applications also seal pores in materials such as wood and concrete to reduce the seepage. If appropriately applied, primers will upgrade the life of the paint, and furthermore improve the appearance.

Can topcoats be used as a primer?

Topcoats cannot be used as a primer or vice versa. As both the coats are designed for different applications and purposes, so accordingly their formulation compositions are quite different. A general metal primer has a combination of various anti-corrosive pigments. Therefore it is very difficult to attain an excellent finish on the primer. Hence, primers generally have a rougher finish than topcoats and are usually available only in matt finish. Secondly it is not possible to have many shades in primer due to the presence of anti-corrosive pigments and other ingredients. However, in highly porous concrete sometimes a clear coat is used as a primer.

How many coats of topcoat are required to cover a dark primer?

Generally the shade of primer is selected in combination with the topcoat, which is to be applied over it. Below are the general guidelines that need to be kept in mind before selecting the topcoat and the number of coats:

  1. Light grey shade of primer is considered as a neutral shade that can cover a variety of shades of topcoat. 
  2. Red oxide shade for primer is generally considered for darker shades of topcoat. 
  3. Off-white shade for primer is generally considered for yellow or orange shade of topcoat.
  4. Total film thickness necessary
  5. Ensure sagging does not take place

Considering the low film thickness of topcoats, the general recommendation is 2 coats of the topcoat. For your specific application, please contact your nearest Grand Polycoats sales office.