Wood primer

From CosplayWiki

There are several reasons why using a primer before painting wood is mandatory. First of all, wood is very porous and will absorb moisture from paint, drying the paint prematurely. As some paints undergo chemical reactions during the process of curing (for example, latex-based paints actually polymerize when curing), they depend on water or solvent being evaporated, not being absorbed. A layer of primer will help the paint to undergo proper complete curing cycle. Second, unless you are painting wood to a much darker color, if you don't use primer, several layers of paint (as many as 8) will be needed to completely obscure the wood grain, and ensure even color.

In general, primers are anywhere from 10% to 75% cheaper than paint, and using a primer prior to painting simply makes sense economically. Getting an even color is also a concern, as wood is not uniform, and will absorb paint differently. If you want for the wood grain to show, you really would be better off using wood stain, not paint. And lastly, if wood is exposed to moisture, a thin layer of paint will still be water permeable. The end result will be warped parts, mildew, and dry rot. Thus, make sure to use a primer every time wood is exposed to the elements.