Spot problems early
Paint thinner, the great scratch detector
As you prepare to finish, you'll find that paint thinner is one of your best friends. What you see with a wet coat of paint thinner is what you will see with a finish, as shown in the photo. Blotches, scratches, glue spots, and other flaws will leap out at you, just begging to be fixed.
Paint thinner evaporates fairly quickly and leaves no contaminant on the wood. You can use any kind of finish without problems, after the thinner dries. The next page tells you how to handle three typical defects.
Typical defects solved
Scratches. If you see scratches or machine marks from sanding, you might not have sanded adequately with your last grit of paper. Re-sand, and check again. If you still find prominent scratches, go to the next finer grit, and sand the wood thoroughly once again.
Blotches. If you examined a board with a microscope, you would see many tiny holes called "pores." Woods with fine pores, including pine, cherry, birch, and maple, tend to blotch when stained. This uneven coloration is a result of variations in the density of the wood. Anything put on the surface tends to absorb more in the softer areas than in the harder areas of the wood. The greater the absorption, the darker the color. Your paint thinner will reveal potential blotching problems.
To avoid blotching, make a homemade conditioner, as shown in the photo and described at the bottom of the page. Most commercial conditioners, as well as homemade mixes using lacquer or varnish, will leave a slight amber cast.
For a colorless conditioner, or one that will be coated with a water-based finish, use a thin coat of clear shellac. You'll find premixed, canned shellac at most home centers and hardware stores. This product is too thick for a conditioner, so mix 1 part shellac with 4 to 5 parts denatured alcohol to make the amount you need for your project.
Glue spots. Whether it's due to normal squeeze-out or an unnoticed drip, dried glue will produce an unsightly spot in the finish. Let the glue start to set, then scrape it off with a sharp blade, and wipe the wood with a damp rag.
Again, paint thinner will make glue spots visible before you finish. If they show up after you have applied stain or a topcoat, you'll have to scrape or sand to remove them.
You can buy conditioners that even out the density of the wood and minimize blotching. However, you can accomplish the same thing with a homemade concoction. Unless you're planning to use a water-based finish, condition blotch-prone woods with a highly diluted coat of the final finish prior to staining.
For example, if you've chosen an oil-based polyurethane as your finish, mix one part poly with five parts paint thinner. Apply a liberal coat of that mix and allow it to dry. Lightly sand the wood with 220-grit sandpaper, and you have evened out the density. You'll leave the mix in the softer areas, and sand it off the harder areas. Always test your mixture and procedure on a scrap piece before trying it on the actual project.