Selecting The Right Undercoating Before Painting

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What is Paint

Before we discuss undercoat, let’s talk about paint. Paint consists of three things; pigments, which is the color, solvents to make the pigments a liquid form so it can be sprayed, and binders, which acts like a glue to hold it all together. There are also additives like alu
inum flakes to make metallic colors or mica to make pearlescent colors. Each color differs with the amount of pigment it has. So some paints are going to cover better than others. The less pigment in a color results in a more translucent color.

Color Matching Starts with the Undercoat

Getting a correct color match begins with the undercoat due to many colors are translucent. This basically means that you can see through the colors. Therefore, the color is going to be different shades depending of the shade of the undercoat. For example, a darker undercoat is going to produce a darker finished color and a lighter undercoat is going to produce a lighter finished color.

Hiding

Traditionally undercoating was designed to help achieve color hiding faster. For example, one coat of high pigment undercoating will reduce the number of coats of base coat needed to achieve full hiding. However, with many of the more translucent colors, the undercoat actually plays a part in the overall color match so determining the correct shade of undercoat is crucial.

What Can Be Used For Undercoat

Depending on the product system being used a primer sealer, primer surfacer, or even base coat can be used to provide the correct gray shade undercoat.

Primer Sealer

Primer sealer is a primer that is not sanded after applied. Once the surface has been prepped, masked, and ready to spray, then one to two coats of primer sealer can be applied to the surface. The number of coats may vary depending on the brand of material you’re using. Once applied allow the primer sealer to flash off, usually ten to thirty minutes, then apply base coat on top of the primer sealer.

It’s important to read the technical data sheet before using any paint materials to determine what their recommendations are for number of coats, flash time, spray gun settings, etc. These may vary depending on the specific product being used.

Primer Surfacer

Some paint companies also make primer surfacer with the correct gray scale or that can be mixed together to get the correct shade. Some primer surfacers can be tinted with color to produce the correct gray scale. However, primer surfacer must be sanded. If this method is used there must not be an sanded through areas, as this will provide an inconsistent undercoat shade. If this method is being used the surface is sprayed with primer surfacer, block sanded, final sanded, then it can be masked off. Once masked off base coat can be applied straight over the primer surfacer.

Not all primer surfacers can be tinted. So it’s important to read the technical data sheet for the material you’re using to determine if tinting is recommended. Also for the number of coats, flash time, spray gun set-up, etc.

Base Coat

Base coat can also be mixed to achieve the correct gray scale. Once the surface has been prepped, masked, and ready to spray, apply the correct gray scale base coat. Allow to flash, then apply base coat top coat. This can be helpful with some brands of waterborne paint. I have a PPG video below to help explain this process better.

How to Determine Which Shade of Undercoat to Use

When looking up a paint code the manufacture should recommend the undercoating. Generally a light color will require a light undercoat and a darker color will require a darker undercoating. However, this may vary to different paint companies and the specific products used. It’s important to read the technical data sheet to determine what is recommended for the specific products being used.

Video – PPG Explains Using Base Coat For Translucent Colors