Professional painters will tell you that wood should be sealed before painting. This is good advice. While paints do provide a measure of protection to wood, paints are not waterproof, and we know that water is the enemy of wood.
A sealant/primer is a wise first step in finishing wood after it has been sanded, smoothed, and cleaned. When using acrylic paints on wood, painters actually will recommend a rigorous preparation before the first coat of paint is applied. That prep includes 2 coats of sealer and 2 coats of primer.
Acrylic paints are often used by painters on canvas, although acrylic paints can also be used on glass, plastic, metal, stone, fabric, and leather. For this piece, we are only concerned with acrylic paints on wood.
As we know, wood is porous. It will soak up paint into its pores, which may affect its color. While acrylic paints provide a small measure of water resistance, they are by no means waterproof. A sealer will offer a good measure of protection for the wood from water and moisture, and also protect against Support Induced Discoloration (SID), which is unique to acrylic paints. The impurities contained in the suspension (support) can cause an amber yellow discoloration to light-colored acrylics.
A primer will allow acrylic paint to apply easier. The combined protection of sealer to protect the wood and primer to facilitate application of the paint set the wood up to better receive acrylic paints and allow lighter color paints to appear more vibrant.
This prep work before painting protects the wood. But what about protecting the paint?
In This Article
What is Acrylic Paint?
Acrylic paints differ from latex paints in that acrylics are chemical-based with water (pigment suspended in acrylic polymers, silicone oils, defoamers, and metal soaps) that make it more elastic than latex paint, and latex paint is simply water-based.
As noted earlier, acrylic paints are a common choice for artists on canvas. They are water-soluble and clean up easily with soap and water. They do provide a higher degree of durability and moisture resistance, although they are not waterproof.
They are more expensive than latex paint, not as easy to work with as latex paint, and more prep is suggested for their use on wood. Acrylic paints are not as easy to wash as latex paints, and on some surfaces will require a specialized cleaning agent to wash.
It is because acrylic paints are not waterproof, and can with sufficient force be cracked, that we now turn to protecting them just as we protected the wood before application.
What is Polyurethane?
Polyurethane was invented almost 100 years ago and employed in many different applications, including shoe soles, mattresses, and foam insulation, along with as a wood finish. It is non-toxic, food safe, and with several applications will create a hard and durable protective surface. It is a very popular choice for wood finishes today, replacing varnish as the most common in woodworking projects.
There are oil-based polyurethanes and water-based polyurethanes. Each offers a hard and durable finish to wood that protects against water and moisture. Oil-based versions take longer to dry and can develop a slight amber tone over time. Water-based versions dry quickly and also offer a hard and durable finish that is waterproof. It does not develop any color tone over time, maintaining its clarity as a finish.
Neither version penetrates wood, instead forming a protective film over the wood. Each is a plastic that is applied as a liquid, and when it is fully dried is a solid plastic sheet.
As you can no doubt tell by now, polyurethane can be applied over acrylic paint. In fact, it’s a good idea to do so. The sealer protects the wood; the primer makes applying the paint easier and with better adhesion; the paint adds the color and character to the wood; and the polyurethane then protects the paint.
How To Apply Polyurethane Over Paint
Standard practice for applying polyurethane over paint is the same standard as applying the sealer and primer that came before the paint. Of course, you will wait until the acrylic paint is fully dry before beginning with the poly top coats. Follow the instructions on the can with regard to drying and curing time. Then:
- Clean. Greasy surfaces prevent polyurethane from adhering well. Be sure to clean the painted surface well. A degreasing agent could be used on newer paint jobs, something like a dish detergent with a degreaser, or a product like trisodium phosphate (TSP) will also work well on older paint jobs. It’s better known as a food additive, but it’s also an effective cleaner. Wear goggles and gloves if using TSP.
- Sand. Use a light sandpaper, a 220-grit, to remove any surface irregularity on the paint, and to give a texture for the polyurethane to adhere to when applied.
- Vacuum and wipe. Vacuum the surface and wipe with a tack cloth to remove all dust so the polyurethane goes on smooth and clear.
- First application. Apply a thin coat of polyurethane to the paint surface and allow it to dry fully. You can use a brush, a roller, a foam brush, or even a spray can to apply. Just be sure to let it dry fully.
- Sand again. Another light touch with a 220-grit sandpaper, making sure to remove any dust dots and brush marks.
- Clean again. Vacuum and wipe again, removing all dust from the surface.
- Second application. Again, a thin coat, allowing it to dry completely before considering a third coat.
If the second coat gives you the smooth finish you were looking for, additional coats may not be necessary. You should always apply 2 coats, and a 3rd wouldn’t hurt. Simply follow the same procedure as before – sanding, cleaning, applying – for each additional coat you decide to use.
Will Polyurethane Ruin Acrylic Paint?
No. How’s that for a simple answer?
It will not ruin the paint. The reason to use a polyurethane over acrylic paint is to protect the paint. The wood is already protected by the primer, but the paint will welcome the protection the poly will provide. It dries to a hard and durable film coat of plastic, and the paint will thank you.
However, we do recommend you use a water-based polyurethane. The clear, glossy coat will let the paint color shine through, and the water-based version will not discolor over time like the amber tone an oil-based poly will develop. The water-based poly will dry faster, too.
Polycrylic Over Acrylic Paint
Polycrylic is another top coat product that can be used over paint. It has an acrylic base, and polyurethane is added to it for better adhesion and durability. It, too, forms a solid film seal and will protect wood from temperature fluctuations, water and light.
Polycrylic is less smelly than polyurethane and is thus a bit easier to work with on your projects. It can take a longer time to cure, though, before your project can be used. It is not as durable as polyurethane alone, too, and can discolor with amber tones over time.
Minwax Polycrylic, however, is a clear top coat finish that does not discolor over time. It comes in several sheens, including satin, semi-gloss, and glossy, dries completely clear and quickly, and maintains its clarity.
While there are videos on poly top coats over paint, we were unable to find one specifically on point with acrylic paints for woodworking projects. However, the numbered steps we offer above are clear and easy to follow, and are common with applying poly coats generally, so there really is no mystery to it.
The short answer to the article’s question is yes, you can put polyurethane over acrylic paint. In fact, we recommend it. Protect the wood with a sealer; prime over the sealer to give the paint something to adhere strongly to; and, then protect the paint with a poly sheet of plastic.
It dries to a clear finish, allowing the vibrant color of the acrylic paint to shine through, and will keep the paint safe from temperature fluctuations, water, and light. It’s a good combination, and your project will turn out with a professional appearance that will last for years.