Bob Oswald replies: Sanding sealers are used to made soft woods or wood that absorb in irregular ways, more even in the way they absorb stain. Commonly soft woods like pine and poplar, and hardwoods such as Cherry and Maple, are very blotchy. They have harder and softer regions in the wood and absorb less or more stain accordingly. Burl type woods like maple and walnut also have wild variations in the rate at which they absorb stain.
Sealer’s partially fill the softer areas, since, of course, they absorb more of the sealer than the harder areas. You don’t want it on so heavy that it puts a ‘layer’ over the wood preventing stain from penetrating. So sealers are very thin and are intended to be wiped on and off, allowing the softer sections to assume more the properties of their harder neighbors. I worried about it oversealing for a while and finally just dived in, figuring I could sand to bare wood if necessary. You don’t need to worry, they work. As usual, of course, follow the can directions and test on a scrap piece of wood.
There are a number of products on the market, Zisner’s SealCoat, General Finishes Seal-a-Cell and Pre-Stain Conditioner and Crystalac, to name a few. With General Finishes you typically wipe it on, wait a few seconds and wipe it off, then go right to your stain. Crystalac, a jelly like material, is best applied by brushing it on, vigorously squeegee it into the wood and removing absolutely as much as possible, then letting it dry overnight and sanding it gently at your finish level, typically 220 grit. I had a very stubborn walnut burl that was incredibly blotchy. After my third strip and try again, this product made it a perfect finish.