Painting 101: Choosing The Correct Automotive Primer And Sealer

Thanks to the availability of Internet information, anyone who has ever sprayed a coat of paint can post their expertise online. This has led to a lot of bad or outdated information. People toss around terms like primer, sealer, two-part paint, tri-coat, lacquer, single-stage, and it can get confusing.

Let’s start by dispelling one of the most common misconceptions. Many clients ask their painters to use lacquer. They think they will get a better paint job by using lacquer. This is not true. Ever see a classic car with a paint surface covered in tiny cracks? That’s because it was painted with lacquer. Unless you’re painting a 100-point concours restoration, you shouldn’t be using lacquer. Modern paint is easier to use, looks better, and is much more durable that any lacquer paintjob.

How Paint Works

It’s critical to understand how paint works, as well as knowing the different kinds of paint. New-technology paint works differently than old lacquer. Lacquer was made with very strong solvents. While this allowed solvents to reach deep into the surface to form a strong chemical bond, it also created a variety of serious problems like crazing and wrinkling. This meant when something when wrong, you had to remove all the paint right down to the metal.

 

primer

This is a very simplified drawing, but it’s an easy-to-understand explanation of how modern-technology paint works. The paint is sprayed onto a sanded surface, and the sanded surface provides “teeth” to help the paint stick. The solvents in the paint, carry the pigments and resins to the surface, then as they evaporate out, the layers bond together.

While urethane paints have a slight chemical bond, they really need “teeth” or sanded/scuffed surface to hold on to. This is why it is very important to understand the product you are using and what it needs. First, you need to know how to prepare the surface. What grit of paper do you need to use? How long can the product sit until it needs to be sanded before painting over it? This is called the “window.” All of this information can be found in the product’s tech sheet.

The Paint Windows

There are two paint “windows” that affect the outcome: The recoat window between coats of paint, and the recoat window before painting another round. So, what is a “round” and how is it different from a “coat” of paint? Say you’ve done the bodywork and are ready for primer. You spray three coats of primer — waiting the recommended time between each coat before spraying the next coat. These three coats are one round.

primer

Here’s part of my library of tech sheets. A tech or P sheet explains everything you need to know about the product. What it can be sprayed over and how to prepare the surface, the kind of spray gun that should be used, the air pressure needed, how the coat should be applied, recommended number of coats, time between coats, recoat windows, and dry times. You’ll also learn how it should be mixed and what should be mixed with it. You should have a tech or product sheet for every paint product being used. Read through the sheet and follow what it says.

Then, the primer cures overnight. This is the window between rounds. Then, the primer is sanded and one coat of sealer is applied. The tech sheet for the sealer says it needs to dry for 30 minutes (another recoat window), and then one round of three coats of basecoat-color can be sprayed. The basecoat dries for one hour (recoat window), and two coats of urethane clearcoat is sprayed.

The tech sheet will tell you the recoat windows for the product. Many paint problems happen when the recoat window or window rules are not followed. Remember, film thickness can also affect the recoat window. The thicker or heavier the coat, the longer it takes for the solvents to gas out.

Avoiding Delamination and Adhesion Problems

Follow the recoat rules on the tech sheet. If you go past the recommended time for recoat, take the time to sand or scuff that surface! Here’s what will happen if you cheat by just a small amount of time. If you’re doing a one-color paint job, sure the paint will look great — at first. Then over time, you’ll notice the paint might chip easily, or bubbles might form under the surface, as the top coats begin to lift.

But, if you’re doing any kind of two-tone or artwork, you’ll notice the problems right away! Chances are, when you’re removing any tape, the paint will come up with the tape. Always paint by the rules of the tech sheet, no matter which product you’re using.

Most paint products are designed to be sprayed in certain conditions. For instance, it’s not a good idea to paint when it’s very cold or very hot. Standard conditions are temperature, humidity, and air flow under which an automotive paint product’s dry time, cure time, pot life, and all general performance characteristics are determined. This information is found on the products tech sheet.

The 15-Degree Rule

What happens when you try to paint in colder or warmer temperatures than recommended in the tech sheet? This is where the 15-degree rule comes in. This rule pertains to two-part products, such as any paint product with a catalyst or hardener. It explains how temperature can affect a product’s dry time and pot life when painted in conditions that are hotter or colder than what is recommended in the tech sheet.

  • For every 15-degree increase in temperature above standard conditions, a product’s dry time and pot-life may be reduced by half.
  • For every 15-degree decrease in temperature below standard conditions, a refinish product’s dry time and pot-life may be doubled.

 

Here’s a timeline for the average paint job. For example, let’s say sealer has been sprayed and basecoat will be applied next. This timeline is based on a 70-degree shop. This is just an example, as all products will be different.

The term pot-life applies to 2K (two-part) products. The pot life is the amount of time a mixed 2K product can be used after the two parts — usually paint and hardener — have been mixed together.

What is VOC?

VOC is one of those terms that gets tossed around and folks don’t know exactly what it means. Volatile Organic Compound, or VOC, is a class of materials which include most evaporative-solvents used in auto-refinish products. VOC is found in nearly all refinish products. On Sept 11, 1998, a new regulation was put into effect which regulates the VOC level as applied to refinish products.

Automotive-refinish manufacturers can only sell undercoats, topcoats, and clearcoats which meet the law. Some states like California, have enacted VOC regulations that are stricter than the national rule. Paint companies have responded by creating water-based paint and solvent products that are VOC compliant. Later in our Painting 101 Series, we’ll cover understanding and using waterborne and water-based paints.

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This graphic gives a basic look at each type of product in a refinish system and shows that each one contains VOCs. VOC is measured in pounds of VOC per gallon. VOC information for PPG products can be found on the product bulletin.

The Undercoats

A paintjob is only as good as its foundation. Don’t use a cheap primer under expensive top coats. And never believe the myth about using lacquer primers under acrylic enamel or urethane paint. Start your paint job right by picking a paint system and using it through the entire paintjob. Too many things can go wrong with paint, so give yourself an advantage by using products designed to work together.

Epoxy Primers

Think of epoxy primer as the foundation for your paint. These primers create a harder, more chemically resistant coating. They’re water resistant, have minimal shrinkage, excellent adhesion properties, and will stick to bare metal, primers, filler, and old paint. For example, you can spray epoxy primer onto a surface that’s been sanded down to bare metal, repaired with body filler, and then feathered into the original paint. Understand that epoxy primer will not stop or neutralize any rust that has started or is already present. Make sure to remove and treat any corrosion before applying primer.

Refer to the tech sheet for the time the epoxy primer can be recoated before it needs to be sanded or scuffed. Missing that window is one of the biggest mistakes painters make. On the other end, pay close attention to the time the product must dry before applying paint over it. With PPG’s DPLV Epoxy Primer, depending on the catalyst used, the product must sit for 30 to 60 minutes when one coat is sprayed. But, if two coats are sprayed the wait is 60 to 90 minutes. Yet, when applying over body filler, the wait times are very different — one hour wait time for one coat. If spraying two coats, the product must dry overnight before applying over body filler. Wet sanding epoxy primer is usually the best way to prep it because epoxy has a tendency to clog paper when sanded dry.

Polyester Primers

Polyester primer is a very thick two-part product and is used when there’s a good deal of bodywork. Poly primer is great for using over bare fiberglass and epoxy. It’s a high-solids product and has minimal shrinkage. It sands easily and is perfect for getting those wavy panels straight. Think of it as a sprayable body filler. Using this product can save hours of time. Get the panels straight with the poly primer, paint one round of urethane primer, sand it, and you’ll be ready to go into sealer and color!

primer

Evercoat’s Super Build 4:1 is a two-part polyester primer surfacer with great filling capabilities to help get the surface flat. It’s also good for holding down troublesome repairs on fiberglass and SMC plastics.

After spraying the poly primer, block-sand it with 80-grit to flatten the surface. Then do most of the sanding with 180-grit, and finish with 220. This will provide the flat surface you want. The courser grits work better for getting the surface flat. They carve more effectively, scraping away the high spots. The finer grits tend to remove less of the high spots, passing over the bumps.

Never wet-sand polyester primer! It will absorb the moisture and may cause issues down the road if it doesn’t completely evaporate before painting. While polyester primers are great for filling, they can be porous. Once you’re finished sanding the polyester primer, use a urethane primer or a sealer over it. Urethane primers are a better surface for basecoats. The urethane primer is much denser and seals the polyester surface. Always check the product sheet to know what works best for the poly primer you’re using.

Primer Surfacers

A quality urethane primer-surfacer will do a good job of leveling out the transition from plastic filler to metal, as well as filling the sanding marks in the filler. But, don’t try to use primer-surfacer where plastic filler or polyester primer can be used. Keep to the recommended number of coats. That information — plus recommended air pressures and wait times — can be found in the product sheets. Each product is different, so make sure to read through the product sheet before priming. Never leave a two-part primer sitting in the spray gun. Most catalyzed primers start to harden in about an hour. Let each coat flash (dry dull) before applying the next.

primer

K36 is a standard-build urethane primer. K38 is a high-build primer. K201 catalyst can be used with either one. They are both great quality primers, sand beautifully, and are very easy to use.

Urethane primers come in a standard and high-build variety. Urethane primers also have great filling qualities. Follow the instructions for mixing and flash times (dry times in between coats). Primer surfacers shrink when flash times (usually 5 to 10 mins) are ignored. Do not use a hardener meant for one product with a different product.

If preparing the surface for sealer and topcoat application, use 320-grit when block-sanding urethane primer-surfacer. It’s rough enough to sand it quickly and flatten out any small imperfections. Then, wet-sand the surface with 400-grit. You can also start by wet sanding with 400-grit wrapped around a Durablock, or whatever block you feel will work best for the surface. Spraying a light guide-coat on the surface prior to sanding shows the high and low areas, and helps keep the body lines straight as you sand.

Sealers

Years ago, I never used sealer. Now, I don’t even consider painting without first spraying sealer. Think of sealer as an insurance policy. Sealer takes care of many problems. It creates a bond between properly sanded primer and the basecoat paint. Sealers help the paint to stick, and can reduce the chance of having the topcoat paint run. Sealer fills those light sand scratches and can diminish very slight imperfections. The sealer also helps to “seal” the surface. The edges of the layers that were sanded-through tend to heal under a coat of sealer. If sealer was not used, they might show up through the topcoat.

primer

Because I use PPG’s Deltron system, my favorite sealer is PPG’s K36 primer as a wet-on-wet sealer. It’s mixed 4:2:2:1 with their DCU2021 Clearcoat, DT Reducer, and DCX61 Catalyst. I spray one wet coat, wait about 30 minutes, then continue with the basecoats.

Sealers also help the basecoat to flow. But, since sealer isn’t sanded, it will leave more texture if it’s not sprayed flat. If you get a grainy surface with the sealer, that surface will show up in your basecoats. Wait times can be critical with sealer. Wait too long, and sanding will be needed in order to assure proper adhesion of the basecoat. Too short a wait time, and solvents might be trapped. Sealer adds to the cost of the paint materials, but the troubles it prevents makes it well worth it.

Colored sealers can also be a big help. A white sealer under a light basecoat, helps the topcoat to have better coverage. Some sealers come in a range of colors, so check your paint brand for its sealer choices. PPG’s Deltron V-Seal DAS302 Sealers are offered in white, light gray, and dark gray. There are some sealers than can be custom tinted to better match the topcoat color.

An Easy Way To Avoid Many Painting Problems

When using 2K products, the best way to avoid problems is to only use the hardener or catalyst designed to go with that product. It doesn’t matter if your buddy tells you, ”Sure use that, I do it all the time.” You take a chance when using a hardener that goes with a different brand of paint or filler. Chances are, something is going to go very wrong, and you will never know the real cause of the problem. If you do not have the correct hardener to go with the stuff you’re using — stop, and wait until you get the right hardener. It’s never worth it to use questionable hardener!

Next time, we’ll take a close look at basecoats, and teach all you ever wanted to know about solid colors, metallics, candy, flake, and pearls. What is candy color and why does it need a base? The best way to get that pearl color to glow. Why are black and white two of the hardest colors to properly spray? What is the trick to spraying a flake paintjob with minimal coats? And there will be some sweet candy paint photos to drool over. Understanding how paint works is the key to getting that perfect paintjob!

Article Sources

About the author

JoAnn Bortles

JoAnn Bortles is an award winning custom automotive painter, airbrush artist, certified welder/fabricator, author, and photo journalist with over 30 years of experience in the automotive industry.
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