How to Paint Cardboard Without Warping?
When it comes to craft projects, cardboard is an excellent material. It is lightweight, easily manipulated, and often just lying around, waiting to be sent off to the recycling center unless otherwise repurposed.
In conjunction with a pair of scissors, a crafting knife, and some glue, it is perfect for creating 3D models, toy weapons, or Halloween costumes.
It can also serve as a functional canvas for a painting, poster, or sign, though this is where the difficulty arises! If you try to paint a large area, it will invariably warp as the paint dries.
The bad news is that this is the nature of the material itself; cardboard is, by its very nature, prone to warping when exposed to moisture.
The good news is that by preparing the surface and properly applying the correct paint, you can minimize the warpage.
Whether you are setting out to make a poster, trying your hand at painting a landscape, or putting the finishing touches on an awesome cardboard costume, we’ll explain how you can apply paint to cardboard without it warping.
1. What is cardboard?
There are two types of materials that we commonly call “cardboard”: corrugated fiberboard and pasteboard.
Corrugated fiberboard is a fluted material that is typically used to create shipping boxes.
As Jay Singha and S. Paul Singh have explained, its characteristic ridges and furrows provide an extraordinarily lightweight yet durable structure:
When a piece of corrugated fiberboard is placed on its end, the flutes form rigid columns, capable of supporting weight without compressing. This allows many boxes to be stacked on top of each other.
When pressure is applied to the side of the board, the space in between the flutes serves as a cushion to protect the container’s contents, thus providing shock protection.
Unlike corrugated fiberboard, which is designed to have such fluted voids, pasteboard is solid, without voids, and is constructed by effectively gluing sheets of paper together or pressing plies of paper pulp together.
This is the type of cardboard used to make cereal boxes. If you carefully work at the edge of a piece of pasteboard, you can peel apart one or more of the constitutive plies.
Both types of cardboard corrugated fiberboard and pasteboard are forms of paper, and paper is produced by extracting cellulose fibers from wood pulp.
In effect, a piece of paper is a matrix of randomly overlapping cellulose fibers.
2. Why does cardboard warp when it’s painted?
Cardboard is composed of multiple layers of paper, and each layer of paper is composed of cellulose fibers. Cellulose fibers become elongated when they take in moisture and they contract as they lose moisture.
When the paint is applied to one side of a piece of cardboard, some of its moisture is absorbed by the paper fibers.
This causes them to expand. As they do so, the matrices formed by the fibers also shift and expand, and this makes the painted surface look wavy and buckled.
As the paint dries, the moisture in the fibers also dissipates, causing the individual fibers to constrict and the matrices formed by them become compressed.
As the painted surface constricts, it is met with the tension created by the adhesion to other plies of paper or (in the case of corrugated fiberboard) to the corrugated flutes and the other wall of the cardboard. This results in a warping of the board.
Pasteboard will warp in a seemingly random fashion: the corners may warp in different directions and parts of the board may “pop” up while others become depressed.
By contrast, when corrugated fiberboard is painted, it will generally warp in the direction opposite of its flutes, since they provide structure, which prevents warping in the direction of corrugation.
Because of this added structure, the painted side will become concave. If the paint was applied evenly, the concave formation will be more or less even as well.
It is difficult to totally prevent the cardboard from warping when painting it, but the effect can be minimized by securing the cardboard prior to painting, properly preparing the surface, and then applying thin layers of an appropriate paint.
3. Minimize warping by preparing the surface
As you set out to work on cardboard, try to find a workspace that is dry, with very little moisture. Basements, attics, and garages tend to be highly humid, so they are not an ideal environment in which to work with cardboard.
Make sure any surfaces you are working on are free from spills, condensation, and so forth.
If you want to use cardboard in place of a traditional canvas for painting, you should begin by cutting the cardboard to your desired size.
Next, find a firm surface to which you can tape the cardboard. If you have a flat piece of plywood or a board that is slightly larger than the cardboard, that would be ideal.
Use long pieces of masking tape to firmly secure each side of the cardboard to the flat surface.
Cover at least ¼” of the cardboard with the edge of the tape and then press outward, away from the center of the cardboard as you press the tape onto the supporting surface.
The tension created by the tape will help offset the tension created on the surface of the cardboard as the paint dries.
If you are painting a 3D cardboard creation or otherwise cannot flatten and secure the cardboard, don’t worry. This is only one part of the surface preparation, and if you have to skip it, that is fine.
The next step is to prime the surface. Cardboard is highly porous and if you apply paint to it without first priming it, most of the pigment will simply get absorbed into the fibers.
While the paint is still wet, it may appear the way you intend it; however, as it dries, the vividness of the color will fade. Priming creates a less porous surface and thereby prevents the paint from becoming absorbed by the cellulose fibers.
Regardless of whether you ultimately want to take a brush to your cardboard or plan to use spray paint, the best primer is an acrylic-based gesso. Gesso (from the Latin gypsum) gets its name from the chalk or plaster that is suspended in a glue-type base.
It has long been used to prepare the surfaces of canvasses and panels for painting, and it works well on cardboard. Apply a thin, even coat using a flat paintbrush.
Let it dry and then add a second thin coat, applying it in the opposite direction.
While it may be tempting to start painting as soon as the gesso is dry to the touch, you should let it cure for 24 hours.
At that point, check to ensure that the primer has been evenly applied. If there are any darker areas (that is, areas where the cardboard is showing through), continue applying thin coats until the cardboard is completely covered.
After achieving a complete and even coating, you can check to see if the texture is appropriate for your needs.
The particles of chalk or gypsum in gesso are intended to create a rough surface since this allows for more precise brushstrokes.
However, if it feels a bit too rough, you can use a fine-grain piece of sandpaper (600-grit or more) to smooth it down.
If you choose to sand, do so lightly and carefully, being sure not to sand the gesso completely off.
4. Selecting a paint
Certain kinds of paints are simply not appropriate for use on cardboard. Most obviously, watercolors are not a good choice, particularly if you have a set of “pan” watercolors the hard pigment disks that require rehydrating with water.
When painting on cardboard, you need to limit the amount of moisture introduced to the cardboard as much as possible; unfortunately, with watercolors, this is almost impossible.
You can try using tubes of watercolor paint and apply it using dry brushes, but this is an expensive proposition since such tubes are much more expensive than pan watercolors.
If you plan to paint even a modestly sized piece of cardboard, you are better off putting the watercolors away.
The best type of paint to use on cardboard is acrylics. Not only are they readily available from any craft store or big-box retailer, but they are also very inexpensive. And since they are water-soluble, clean-up is easy.
Spray paints also work well with cardboard, especially if you want to paint a large area. However, as with all spray-paint applications, you will want to ensure that there is good ventilation and that you wear protective gear.
Also, keep in mind that spray paint is messy. Unless you are a skilled graffiti artist, it can be difficult to control the spray.
If there is anything that you want to prevent getting splattered or misted, be sure to cover it with a drop cloth or move it out of the way.
5. Apply the paint
When using acrylics, do not apply the paint directly to the cardboard; doing so introduces too much moisture in a concentrated area. Instead, use a brush or sponge to transfer the paint to the cardboard.
The standard method is best: place dollops of your paint on a painter’s palette or an improvised alternative (for example, a plastic plate or piece of scrap wood) and then lightly dip your sponge or brush tip.
If at all possible, use multiple brushes to avoid having to rinse your brush in water. If you do need to rinse, keep a paper towel nearby and dab the excess water off before dipping it back into the paint.
It bears repeating: avoid introducing unnecessary moisture!
Work slowly and avoid creating over-saturated spots on the cardboard. You should err on the side of caution and make your layers too thin, rather than risk laying down too much paint at once.
You can always add more coats of paint later, if necessary.
Indeed, the best approach, when painting on cardboard, is to plan on applying multiple thin coats, as evenly as possible, leaving time in between for each to dry completely (usually about two hours).
The same principle applies to spray painting: lay down a thin, even coat and let it dry before building up additional layers or coats.
Remember: wet paint is wet, and you want to reduce the amount of moisture on the surface of the cardboard as much as possible.
While it may be tempting to rush and paint as quickly as possible, if you want to avoid warping, you absolutely need to work slowly. So, give yourself plenty of time and plan accordingly.
6. Finishing up
Once you have finished painting and are satisfied with the coverage, let the cardboard dry completely before moving it (or removing the tape).
If at all possible, let your piece sit for a full 24 hours or more. Once you are sure that the paint is completely dry, carefully remove the masking tape, if you used it.
Please note that it may want to stick to the cardboard and if you pull too hard, the tape might cause a tear.
Work slowly and keep a hobby knife at hand in case you need to use the tip to carefully free the cardboard from the tape. If the tape seems to be lifting the topmost cardboard ply, work from the other direction.
Hopefully, if you followed these directions, there will be little to no obvious warp to the cardboard. However, if there is, you can try pressing it out by placing the cardboard between two pieces of plywood and weighing it down.
Additionally, or as an alternative, especially in the case of 3D cardboard objects, you can use stiff material to add structural support to the backside of the cardboard.
Wooden tongue depressors or popsicle sticks, toothpicks, paperclips, or even other pieces of cardboard can be glued to the opposite side to create tension and compensate for any remaining warp.
