You often see cans of solvent finishes such as varnish, polyurethane and lacquer with instructions not to thin them. Manufacturers include these instructions in order to comply with VOC laws in some areas of the country. Adding thinner could take the finish out of compliance with the local or state laws, and manufacturers might be breaking the law if they advocated thinning.
But you can’t do any harm to these finishes by adding thinner. In fact, you can add all the thinner you want, even 99 percent, without causing any harm. You’ll just get a thinner build with each coat, which may cause you to have to apply more coats to get the look and protection against liquids you want.
The advantage of adding thinner (five to thirty percent is usually adequate) is better leveling—that is, reduced brush marking and orange peel.
Keep in mind, however, that if you are in an area of the country with strict VOC laws, you could be taking the finish out of compliance if you add thinner.