Preparing a support properly is the first key to a successful painting.
If you are just painting for fun, you may not think it's important to work with archival materials, but as a framer I often see prized treasures come in with irreparable deterioration over time. If you want your paintings to last, preparing the surface properly in the first place is the key. Certainly you need to consider this if you plan to sell your work.
Acrylic Gesso is absorbent by nature so it acts as a good ground for paint to adhere to. This means that it isn't providing much of a barrier between your surface and your paint. Prior to using gesso, you want to seal your support with a barrier. Traditionally, artists used rabbit skin glue for this purpose. Today it is usually PVA or other chemical "sizing". Even paper is sized to prevent ink and paint from absorbing into the fibres too much.
Why Seal before Gessoing?
Sealer(also known as "size") is a barrier between your support and your paint. When you apply paint to a support it will absorb down into any porous layers. If you are oil painting, this is troublesome as the solvents can slowly deteriorate the wood or fabric support over time. It also causes the glossy oils to sink in further, and show on the surface of your painting as dull patches which can be hard to remedy.
For acrylics it it very important because of "Support Induced Discolouration".
What is that, you may ask? It is usually observed as a yellowing appearance on light coloured or clear areas of acrylic. If you haven't properly sealed the wood, the paint will absorb down into it. Then, as the paint dries by evaporation, the moisture will move back up through the paint film bringing with it any contaminants from the wood. Whites and pale blues will take on a yellow look. Techniques such as paint pouring are especially prone, as you are flooding the surface with a large amount of liquid which takes hours to dry.
There is no real way to fix improperly sealed supports once the problem occurs, so it's important to start your painting off correctly in the first place. Wood panels aren't the only surface with this issue, it has been observed with all supports including canvas or linen. I know it's easy to be impatient to begin your new project, but it helps to have several supports prepared and ready to use at any given time.
How to Prepare a Surface for Painting
First you want to give the panel a light sanding. I use a sandpaper block which I made using plywood, ATG double sided tape, and two different fine grits of sandpaper on each side. Sand with the grain of the wood, and smooth any loose fibres or scratches. I always sand the edges and sides a little to smooth them out as well.
Once you have sanded it to your liking, wipe all surfaces down with paper towel. You can moisten it with water, but I like to use Isopropyl alcohol so that it doesn't raise the grain of the wood. Make sure you remove all sanding dust and any dirt or hairs.
We sell Gotrick brand wood panels, which are made in Canada out of birch wood. They come in different shapes and sizes including rectangles, circles, and triangles.
We will be using Golden Acrylic's GAC100 as a sealer. You can also use Golden gloss medium, though you may see more brush marks as it's a bit thicker. Many artists seem to use Matte medium, though Golden claims that the glossy ones create a better barrier(matte finishes are a bit toothy and absorbent).
Apply your sealer directly to the wood panel, following the grain of the wood. I like to use a large, soft, synthetic brush. Coat the entire surface: front, sides, and back.
Allow the front to totally dry before flipping it over to do the other side. It should be a bit glossy and shouldn't have a tacky feeling to it when it's ready for the next coat. Putting down a piece of parchment paper or non-stick surface helps when flipping it over.
Once the first layer is dry, you can lightly sand any raised grain, wipe clean, and then apply a second layer to all surfaces.
Next you want to prepare the surface for painting. Here I am using white acrylic gesso as a ground. If you want to keep the colour of the wood, you can use clear gesso. There is also black or tinted gesso available.
Try this; Add a little bit of acrylic paint into your gesso to create your own tinted gesso.
Always keep the lid of your gesso(and acrylics) closed as much as possible. Try to keep it out of the threads of the lid, or it will be hard to open later. It can also create little dried chips which leave flakes in your surface. Use a clean painting knife or other tool to scoop out what you need. Don't dip your brush straight into the tubs, as they can go mouldy with excess water or contaminants.
Decide whether you want to paint just the front, or also the sides, and maybe even the back of your support. The nice thing about wood is that you can leave it the way it is and it adds a warm, natural feel to your piece. Black edges are a classic choice, as is white. You can also wrap the painting around the edges for a creative "unframed" look, or just paint it a solid accent colour when it's done.
Make sure you do at least two layers of gesso on the surface you plan to paint. Three is better, especially for oil painting. I like to lightly sand in between layers. To minimize the grain of the wood, I alternate which way I paint on the gesso, and which direction I sand. For the final pass, I lightly sand in a circular motion with fine grit sand paper.
Of course if you want a more textured appearance, you can omit sanding altogether, and even add the gesso on thickly with rough bristle brushes. Sponges or rollers can create interesting textures to work on.
Experiment and find what you like best for your style!