Sanding Sealer is the single most important component to finishing stained trim and doors yet so many people know little about it. One of the more common methods people (DIY) are using today to finish woodwork is one coat of MinWax stain and 1-2 coats of MinWax polyurethane and often they don't bother sanding between coats.
If you want nicely finished woodwork then after you apply your stain to the trim and doors the 2nd step to finishing them is applying sanding sealer. Most sanding sealers dry super fast and can be sanded the same day but for outstanding results its best to wait a day. The sealer becomes more hardened and sands so much easier and smoother a day later.
Use a White China or Black China brush to apply sanding sealer. The White China will provide better results and holds more vs. black china.
It is not necessary to apply sanding sealer heavily. It is important to be sure the area absorbed the sealer enough before you move on. I apply a nice load to an area and work it in quickly and move the rest of the load off into the next area. I want the area saturated but not left heavy on the surface.
6 STEPS to Smooth Woodwork
1) Stain your trim
2) Apply sanding sealer (wait 1 day)
3) Sand the sanding sealer smooth
4) Vacuum with a shop vac (some people like to use a tac-cloth too)
5) Fill nail holes
6) Apply your compatible clear coat.
More on Sanding Sealer here.
SPECIAL NOTE: If you are familiar with ICI Wood Pride Sanding Sealer, Sherwin Williams, Cabot or Benjamin Moore's SuperSpec, the MINWAX Sanding Sealer is nothing like them. The MINWAX is more like sanding polyurethane. Its best to have your wood sanded smooth prior to staining because MINWAX Sanding Sealer will only allow you to sand what's on the surface where the other sealers listed allow you to cut deeper than the surface.
Part 2. More on this Sealer here.