by michael daigle · 9 Jan 2024Table of Contents
So you've decided to tackle painting your house yourself to save some money. Great idea, but before you dive right into slapping a fresh coat of paint on those walls, there's something you need to know about: undercoats. Undercoats are the hidden helpers that make your topcoat of paint look its best. Without the proper undercoating, your new paint job won't have the smooth, professional finish you're after.
What Is an Undercoat in Painting?
An undercoat is a layer of paint applied before the final topcoat. It helps the topcoat adhere better and last longer. For professional painters, using the right undercoat is key to a high-quality, long-lasting finish.
Primer
A primer is a type of undercoat designed specifically to prepare a surface for painting. It seals the surface and provides a uniform base for the topcoat. Primers come in different varieties for different surfaces like drywall, wood, metal or masonry. Applying a primer is especially important when painting over an existing dark color or changing a surface from dark to light.
Sealer
A sealer protects the surface below the paint. Concrete sealers, for example, help prevent stains and moisture damage on porous masonry surfaces. Wood sealers protect against rot and weathering. Sealers are often clear or tinted and provide an impermeable base layer.
Stain blocker
For surfaces with water stains, smoke damage or other hard-to-cover marks, a stain-blocking undercoat is ideal. It prevents stains from bleeding through the new paint. Stain blockers often contain shellac or oil-based primers to block difficult stains. They provide full coverage in one coat.
Tinted undercoat
A tinted undercoat in a shade similar to the final topcoat helps with coverage and provides a uniform base color. Fewer coats of topcoat are needed, saving time and money. For the best results, choose an undercoat specifically intended to be tinted to match your topcoat color.
With the right undercoat or combination of undercoats for your surface, you'll get professional, long-lasting results and a beautiful final finish. Understanding the different types of undercoats and their specific uses is key to quality DIY or professional paint jobs.
Why Use an Undercoat Before the Top Coat?
When it comes to painting, the undercoat is just as important as the topcoat. Here are a few reasons why professional painters always start with an undercoat:
Consistency
An undercoat helps provide an even, consistent surface for the topcoat to adhere to. Without it, the topcoat can look uneven, streaky, or patchy. The undercoat fills in any minor imperfections in the surface and creates a uniform base.
Adhesion
Undercoats are specifically designed to provide maximum adhesion for the topcoat. They grip tightly to the surface while also allowing the topcoat to grip tightly to them. This results in a very durable, long-lasting finish. Without an undercoat, the topcoat may not adhere as well and can start to peel or chip prematurely.
Fewer Coats
Using an undercoat often means needing fewer coats of topcoat to achieve an even, opaque finish. The undercoat provides coverage on its own, so the topcoat has less work to do. Fewer coats mean less time and money spent on a painting project.
Sealing
Undercoats seal and prime the surface, preventing the topcoat from soaking in or being absorbed unevenly. Sealing the surface helps the topcoat glide on smoothly and evenly, and it prevents the surface material from bleeding through or causing stains. Sealing is especially important when painting over dark colors or wood grains.
Protection
A quality undercoat protects the surface by creating a barrier between it and the topcoat. This is important when painting over surfaces that may stain or bleed through like wood, plaster, or concrete. The undercoat protects the topcoat from any chemicals or stains in the surface material.
Types of Undercoats for Different Surfaces
There are several types of undercoats designed for different surfaces. Choosing the right undercoat is key to achieving a smooth, professional-looking topcoat.
Primer
Primer is a base coat that prepares bare or untreated surfaces like drywall, wood, or metal to be painted. It seals the surface and provides a uniform base for paint. For the best results, choose a primer designed for your specific surface.
Sealer
Sealer is a clear base coat used on surfaces that are porous or uneven in texture. It penetrates the surface to create a smooth base, preventing uneven absorption of paint. Sealer can be used on concrete, brick, stone and grout.
Stain-blocking primer
Have you ever painted over a surface only to see the old color bleeding through? Stain-blocking primer is designed to hide stains like water damage, smoke damage or old painted surfaces. It prevents stains from showing through the new topcoat of paint.
Bonding primer
Bonding primer is used when painting over glossy or slick surfaces where regular primer won’t adhere well. It “bites” into the existing finish to provide a rough surface for the new paint to grip onto. Use bonding primer when painting over varnish, laminate or tile.
Oil- or water-based
In addition to choosing a primer for your specific surface, you’ll need to choose between oil-based or water-based (latex) primer. Oil-based primer has strong adhesion and stain blocking but has a strong odor and requires the use of solvents for cleanup. Water-based primer is low odor, easy to clean up but may require an additional coat. For the best results, choose a primer with the same base—oil or water—as your topcoat paint.
Following the proper undercoating techniques and choosing the right undercoat for your surface will ensure a smooth finish and help your paint job last. Take your time—the prep work you do before painting can make all the difference in the final results.
How to Choose the Right Undercoat
The type of undercoat you choose depends on several factors, including the surface you're painting and the finish coat you'll be using. The undercoat creates a solid base for the topcoat to adhere to, so it's important to choose one suited for your specific needs.
Primer
For bare or porous surfaces like drywall, concrete, or wood, a primer is essential. Primer seals the surface and provides a smooth base for paint. For drywall, use a drywall primer. For wood, use a wood primer. For concrete, use a concrete primer. Primer comes in both oil-based and water-based varieties. For the best adhesion, choose a primer and topcoat with the same solvent base.
Sealer
If you're painting over a surface in good condition that's already been painted, a sealer can work as an undercoat. Sealers are very thin and mainly act to create a barrier between the old paint and the new. They come in water-based and solvent-based options. For previously painted drywall or plaster, a drywall sealer is a good choice. For wood, use a wood sealer or wood conditioner.
Tinted primer
For a color change, use a tinted primer under the new finish coat. The tinted primer will make the color transition smoother and require fewer coats of the new color. Have your primer tinted to a shade slightly lighter than your topcoat color. Apply two coats of the tinted primer before the finish coats.
Stain-blocking primer
For surfaces with stains like water damage, smoke damage, or wood knots and stains, use a stain-blocking primer. These primers contain sealants to block stains from bleeding through the new paint. Apply one or two coats of stain-blocking primer before the finish coats, allowing it to dry as directed between coats.
Choosing the right undercoat for your painting project will ensure you get professional results and a finish that lasts. Take the time to determine what type of undercoat is best for your specific surface and needs. Your patience will pay off with a high-quality, long-lasting paint job.
Applying the Undercoat - Tips and Tricks
Applying the undercoat is a crucial step that provides an ideal surface for the topcoats of paint. Take your time and be meticulous.
Use a High Quality Brush
Invest in a brush specifically designed for applying primer or undercoats. A brush with natural bristles works well for oil-based products, while a synthetic bristle brush is best for latex or water-based undercoats. Make sure the bristles are the proper length and stiffness for the job. A quality brush will help you achieve an even, smooth coat.
Thin the Undercoat (if needed)
Check the specifications for your undercoat product to determine if thinning is required or recommended. Thinning the undercoat slightly with the proper solvent will improve its flow and penetration. Follow the directions carefully and test the thinned undercoat on an inconspicuous area first. Thinning too much can reduce hiding power and coverage.
Apply in Short, Even Strokes
Use a brush to apply the undercoat, working in short, even strokes. Extend each stroke 6-8 inches past the previous stroke to avoid lap marks. Work from the top of the surface down using a ‘W’ or zig-zag pattern. Cover the entire area with long, smooth strokes.
Let it Dry Completely
Undercoats need to dry completely before topcoating. Follow the directions on your specific product for dry times between coats. The undercoat must cure to provide proper adhesion for the finish coat. Cooler temperatures or higher humidity can increase dry times. It’s best to err on the side of caution and allow extra time. An undercoat that isn’t fully cured can cause issues with the topcoat like cracking, wrinkling or loss of adhesion.
Patience and proper technique when applying the undercoat will result in a smooth, professional finish. Taking shortcuts or rushing through this step can undermine all your hard work. Follow these tips and your topcoat will glide on like a dream.
Common Mistakes to Avoid When Using Undercoats
When using undercoats, it’s easy for new painters to make mistakes that affect the final results. Avoid these common pitfalls to get professional-looking results.
Not allowing proper dry time
It’s tempting to rush through the painting process, but not allowing adequate dry time between coats leads to problems. Undercoats need at least 4-6 hours to dry before applying the next coat. If you don’t wait long enough, the new coat will wrinkle and bubble. Patience pays off!
Not sanding between coats
Lightly sanding between undercoats opens up the surface of the previous coat so the next one adheres better. Use 220-grit or finer sandpaper and sand just enough to rough up the surface. Wipe away dust with a tack cloth before painting.
Not checking the weather
The weather impacts how fast undercoats dry and cure. Don’t paint if rain is forecast within the next day or temperatures will drop significantly overnight. Cold weather and high humidity also extend drying times. Plan your painting around the weather to avoid frustration.
Using the wrong undercoat
For the best results, use an undercoat formulated for your specific paint type. Latex or acrylic paints require a latex primer. Oil-based paints need an oil-based primer. And when painting over dark colors, use a primer specifically designed for that purpose. The right undercoat makes a difference.
Not priming bare wood
Bare or unfinished wood needs a coat of primer before any undercoating. Wood primer seals the wood and provides a base for undercoats to adhere to. If you skip priming, tannins in the wood can bleed through and the paint finish may peel. For the smoothest results, prime first.
By avoiding these common mistakes, your undercoating and painting projects will turn out looking polished and professional. Take your time and pay attention to the details—the results will speak for themselves!
Allowing Proper Dry Times for Undercoats
Allowing adequate dry time between coats of paint is critical for achieving a professional finish. Rushing the process can lead to a number of issues that end up requiring more work to fix.
First Coat
After applying the first coat of primer or paint, you'll want to wait at least 2-4 hours before adding another coat. This allows the first layer to dry sufficiently so the next coat can adhere properly. If you don't wait long enough, the second coat may crack or peel away from the first.
Second Coat
The waiting period between the first and second coats of paint is the most important. Plan on allowing 6-8 hours of dry time for the best results. The longer you can wait, the better. If possible, apply the second coat the next day for optimal adhesion. Adding a coat too soon will result in uneven sheen, brush marks, and poor coverage.
Additional Coats
For the best finish, you may need 3 coats of paint. Wait at least 4-6 hours between the second and third coats. The final coat will provide an even, professional-looking finish. Be patient through the entire process—rushing at the end won't save much time and will sacrifice quality.
Weather Conditions
Drying times can vary depending on weather conditions like temperature and humidity. Warmer weather will speed up drying while cooler temps and high humidity will slow it down. Make sure to account for the weather when planning your paint project timeline. It's best to paint indoors when possible for the most control over conditions.
Taking the necessary time between coats and considering the weather conditions is essential for achieving a flawless, professional-grade paint job. While waiting may feel tedious, the end results will be well worth the patience. Your efforts will be rewarded with a beautiful, long-lasting finish.
Achieving an Even, Consistent Undercoat
Achieving an even, consistent undercoat is key to a professional-looking paint job. An undercoat provides a uniform base layer for your topcoats of paint to adhere to. Without it, your final finish can look uneven, patchy and lack depth.
To get started, you'll want to thoroughly clean and lightly sand the surface you intend to paint. Remove any loose paint or debris. Vacuum up dust. Apply a primer specifically meant for the surface you're painting, whether it's drywall, wood, metal or masonry. Primer helps seal the surface and provides adhesion for your undercoat.
Use a brush to paint the primer on any trim, edges or hard to reach areas. Then you can use a roller for large, open surfaces. Let the primer dry as directed.
For your undercoat, choose a shade slightly lighter than your topcoat color. A lighter undercoat helps provide coverage in case the topcoat is patchy or uneven in areas. Apply two coats of undercoat, waiting for each coat to dry in between applications.
Use a high quality brush to paint the undercoat on any trim, edges or hard to reach areas. Then use a roller for large, open surfaces, rolling in smooth, even strokes. Work in sections across the surface using a 'W' or 'M' shaped pattern. Extend each stroke into the previous section to avoid lap marks.
Check for any drips, uneven patches or missed spots and touch up as needed between coats. The key is to achieve an even, consistent layer. Once dry, you're ready for your topcoats of paint. With the right prep and undercoat, you'll achieve a professional finish that looks like it was done by an expert.
FAQ - Answering Common Questions on Undercoats
Have you ever wondered why professional painters use undercoats? These primers and sealers provide an important base for your topcoats of paint. Here are some common questions about undercoats answered.
What is the purpose of a primer?
A primer prepares the surface for painting by providing a uniform base. It seals the surface and provides “tooth” for the topcoat to adhere to. Primers also hide imperfections in the surface like stains, water damage or wood grain. Using a primer results in a smoother, professional-looking finish.
Do I need a primer for repainting the same color?
Even when repainting the same color, a primer is recommended. The existing paint will have aged and weathered, and a primer will refresh the surface and provide a good base for the new topcoat. A primer also allows you to make minor repairs or changes before repainting.
What are the different types of primers?
The most common types are latex or oil-based. Latex primers are water-based, low odor and easy to clean up. Oil-based primers provide better coverage in one coat but have strong fumes and require solvents to clean up. Primer selection depends on the surface you’re painting and the type of topcoat. Talk to your local paint expert for recommendations.
Do dark colors require a special primer?
Deep, rich colors like navy blue or forest green often need a tinted primer to provide full coverage in two coats. A tinted primer that closely matches your topcoat color helps prevent the need for additional coats. The primer also helps prevent “bleed-through” of the darker color, especially when painting over lighter colors.
What is a sealer and do I need one?
A sealer protects and waterproofs surfaces like concrete, wood, and stucco before painting or staining. Concrete sealers prevent moisture damage and wood sealers protect against rot and weathering. Using a sealer helps your paint job last longer. Sealers are applied before priming and painting. Talk to your paint professional about whether your surface requires or would benefit from a sealer.
Undercoats provide essential protection and an ideal base for your painting project. Don’t skimp on these important first steps to ensure a professional, long-lasting finish. Ask your local paint expert for recommendations on the best undercoats for your specific needs.
Final Thoughts
So there you have it, a quick primer on the importance of undercoats in professional painting. By now you've learned that undercoats provide adhesion, hide imperfections, and allow you to use less paint overall. They may require an extra step, but the end results will be well worth it. Your walls will look smoother, your colors will pop, and your paint job will last longer. Undercoats: not the most exciting part of painting, but a necessary foundation for a job well done. The pros know that attention to detail with undercoats pays off. Now you're in on the secret too. Happy painting!