If you work with wood, it has happened to you: You cut trim, nail it on and it doesn't fit. Such mistakes result from poor measurements, warped trim or damage from a saw. Problems like this happen even to experienced woodworkers, but they know how to cover them up. Use some tricks that have been around for years, and your trim job will look professional.
Putty crayon quickly camouflages a bad joint. It's formulated to stay pliable and fills gaps, cracks and joints that were cut too short. Use it primarily for finished wood trim as a finishing touch to cover gaps or shortcomings up to about 1/8 inch wide. It is available in colors to match almost any stain. Rub the tip of the crayon with enough force to scrape the putty deep between two mitered corners that don't fit or butt-joints where two pieces don't fit together tight. It will leave a smear on the trim. Wipe the crayon residue off with a clean cloth, and the putty will polish smooth and flush with the surface of the wood. It's also good for nail holes, pits or gouges.
Miter saws can shatter or break the ends off mitered corners, leaving loose wood fibers and rough-looking cuts. If this happens, use a chisel to trim the fibers and splinters or to blunt the edges of the bad cut. Joining the pieces together may yield a raw look to the trim, however, because the chisel removed some of the wood stain. So use a color-matched stain marker to color the repaired ends of the trim. Press the tip of the marker onto bad cuts to release stain right where you need it. Use the marker to hide small gaps between joints that are no wider than about 1/6-inch, and you'll never notice the mistake.
Use wood dough on unfinished trim to fix mistakes or fill gaps. This type of putty, also sometimes referred to as plastic wood dough, is made from real wood fibers. Force it deep into cracks or gaps up to about 3/6-inch-wide. It goes on like soft mud, but hardens like real wood. When the putty is dry, sand the puttied area, and it will blend into the wood like it's not even there. Use species-specific wood dough to match the trim. The putty accepts stain just like ordinary wood.
If you have a serious gap between trim pieces because they were cut too short, use a wedge, also sometimes referred to as a Dutchman splice. To make a Dutchman or wedge, place an identical piece of trim on a miter saw. Cut random slices off the end ranging from about 1/6-inch-thick to about 1/4-inch-thick. Select one that fits tight into the gap but just a bit too wide. Sand the edges of the splice to a bevel shape and apply glue to both sides. Tap the wedge into the gap as far as possible. If you're lucky and have chosen the right wedge, it will fit flush into the gap with the profile of the wedge matching the profile of the trim molding. If the wedge doesn't fit flush, use a sharp chisel to trim the wedge to match the profile of the molding and then color it with a stain marker. If you haven't installed the trim yet, you can glue the wedge directly to the end of the trim and then install it. You'll never notice the small splice.