Products sold as wood conditioner are washcoats usually made from varnish, though I have seen at least one that is an oil/varnish blend. A washcoat is a finish thinned to five-to-ten percent solids with the appropriate thinner. (Finishes are generally supplied with 20-to-30 percent solids.) In industry, the finish used is usually lacquer thinned with lacquer thinner.
Wood conditioners can be fairly effective on softwoods like the pine shown in the accompanying picture. They aren’t as effective on hardwoods such as cherry.
The purpose of the thinned-finish conditioner is to partially seal the wood, which means to partially stop up the pores so the stain, which can cause a blotchy appearance, can’t penetrate as deeply in those areas that will get darker.
To use a wood conditioner effectively, it’s important to understand that the directions supplied by almost all brands won’t produce good results. These directions say to apply the stain within two hours, which is before a varnish or oil/varnish blend has time to dry. So the stain mixes with the uncured wood conditioner and still blotches.
To use a wood conditioner effectively you have to give it time to completely dry so the stain can’t penetrate. With lacquer this is 30 minutes or so. But with varnish and oil/varnish blend this is overnight, or at least 6 to 8 hours.
To significantly reduce blotching, apply the wood conditioner wet to the surface of the wood and let it completely dry. Then apply the stain and wipe off the excess. But notice from the example that with less stain penetration, the resulting color is also lighter.
I have no idea why manufacturers give directions that don’t work. The only explanation I can think of is that they believe you want to totally complete a project on a Saturday afternoon, and waiting overnight for the wood conditioner to dry won’t accomplish this.