The answer to the question posed in the title is not as simple as stating that one is paint meant to cover a surface while the other is stain, manufactured to literally stain the material and even be absorbed into it. This is, of course, the basic difference in the minds of most people who buy one or the other. But, taking the question to the next level, you have to ask how each is used and for what purpose. Both will cover the surface and add new color to it. That much is true.
But, stain is meant to be used when you are coloring and protecting wood that doesn’t have paint or any other opaque coating. However, most of the time you’ll find stains don’t protect the material from moisture as well as paint does. You may also notice a difference in how walking affects the two. Unless you use a special type of paint, foot traffic can change the appearance in short order.
In very general terms, you’ll also find that paint will give you better protection for the long term, and will protect wood from moisture for a longer period of time. A good stain will deliver some benefits in this area, but has the added benefit of allowing moisture to get out of the wood as well. Of course, this means it’s more likely to allow moisture in, especially after time has passed.
Most people will choose paint because of the color options and the difference in gloss available. In addition, paint can be used on a number of different materials, including metal. Because stain is designed to impregnate the wood to a higher degree than paint, in most settings, it doesn’t work as well on slick surfaces or metal. It’s also important to consider the amazing variety of colors offered by paint companies. Stains are produced in some variety as well, but not to the same extent.
It’s also interesting to note how paint and stain might be used on the same surface. This will go a long way toward helping you understand how these products differ. You can, in some cases, put a 100% acrylic/latex over existing stain. Of course, you can also use a stain of solid color. However, make sure you prime the surface when using paint.
To further understand these important differences, think of a solid-color stain as having similar ingredients, but usually in a different quantity. Stain will generally have less pigment than paint. If you buy a true, solid-color stain, it won’t penetrate wood in the same way that some thinner stains would. For this reason, those stains with the most pigment will act more like paint.
Of course, both can be applied with brush, roller, or sprayer. A solid-color stain is a bit thinner than paint, which means you’ll probably have to refinish more often. In many situations, it’s best to strip either coating and sand to the natural-wood state to get the best-looking finish. It’s best to give this subject some forethought prior to beginning your project.