Why do flat finishes need thorough stirring?

I got to the bottom of a can of satin finish and found a thick layer of sediment, which apparently means I neglected to stir it enough. Does it also mean the finish I applied will fail?
—Scott Finney, Montecito, California

Your finish will hold up just fine, Scott, but you might notice that it's glossier than you expected. The sediment in the can is a flattening agent (usually tiny particles of silica) that changes the way light reflects off the surface of the dried finish, resulting in the "satin" look. Without a flattening agent, most finishes dry to a glossy sheen.

All film-building finishes, including polyurethane and lacquer, include flattening agents, which dull the "shine." So stir these finishes thoroughly before and frequently during use to keep the silica particles in suspension. And if you want to rid your project of a too-glossy sheen, simply apply one more coat of satin finish from a new, properly stirred can.