While teaching at the Marc Adams School of Woodworking in Franklin, Ind., this spring, I asked my fellow instructors and colleagues to share their experiences in a problem/solution format. I’ve added a few of my own, too. Follow in our footsteps and you’ll become a better finisher.
MICHAEL FORTUNE: Be sure to sand your surfaces enough, especially when you’re using oil finishes. If your sanding is inconsistent, the oil will soak in and darken up the scratches, where they’ll be easily seen.
To resolve this issue, make sure you go through the proper progression of sandpaper, 120 to 150 to 180 to 220, and maybe even 280, if you need to. Go through each grit size, sanding over the surface three times and be sure you’ve sanded out scratches before moving on to the next grit. Pay special attention to end grain, too.
CHRIS GOCHNOUR: I teach college students, and one of their biggest issues I see is surface continuity. If you have a flat surface, it must be flat. If you have a curved surface, it has to be consistent and smooth. You can’t cover up a bad surface, even with a good finish. The underlying issue will still show through.
So, use a large, flat block on flat surfaces, or, if you’re using a power sander, keep it moving to make sure it’s flat. Don’t tilt it to get a bump out — you’ll make a divot instead. Also, use your hands to feel the surface and stroke the work; by using touch, you’ll pick up any ripples that you can’t see.
If you’re sanding curves, use a large, flexible sanding block. You can make one with 1/8” bendable Masonite with pressure-sensitive adhesive sandpaper stuck to the bottom. Put two handles on the Masonite to manipulate it. Automotive stores sell long flexible sanding blocks for sanding curved surfaces, too.
ALAN LACER: Pay attention to the shelf life of your finishes. Old finishes won’t perform well; they won’t cure or dry properly, which will result in a poor finish.
Instead, you can mix or catalyze your own finishes, which typically last 30 days. Put the date on your label and throw them out in a month. Or, for any shellac or catalyzed finish that you buy, take a big black magic marker and mark the shelf life or expiration date clearly on the can. For oil finishes, using a blocking agent spray like Bloxygen or Extend-It that creates a dry gas blanket to help extend the life of air and moisture sensitive finishes.
ZANE COWELL: I work in Indiana, so humidity is an issue, especially in the summer. Moisture can get trapped in your finish and create a white blush. To avoid this, install an HVAC system, air conditioner, or dehumidifier in your shop. Or, you can mix in special additives into your finishes to alleviate extra moisture. Also, use slow-drying solvents to allow moisture enough time to escape.
MARC ADAMS: One of the issues I see is when people use a lacquer- or water-based finish over a mineral-based oil stain. These finishes are incompatible and the coats don’t bond together.
It’s important to understand the layers of the finish process and how they interact. Also, it’s best to stay with one manufacturer and learn how each product in the line works with the others. Talk to a technical rep; all manufacturers have someone who can share their product knowledge, so you can ask questions and learn more, directly from the source.
GREG JOHNSON: When you work too fast, you’ll rush the process and the applied coats won’t have a chance to gas off, dry completely, and/or cure before the next coat. But there’s an easy solution: take your time! Also, read all application instructions and note the cure times and reapplication windows. Keep a log, record your progress, and pay attention to the clock.
I also keep a separate board made of the same wood or veneer that I prepare along with the piece. As I progress through the steps, I mark off the section I’ve completed and continue down the board until it’s done. This way I have a step-by-step record of the finishing process and can use it for quality control and to match any rework I might have do to.
SCOTT GROVE: My contribution is about air quality. Particulates are in the air, like pollen, bugs, sawdust, hair, dander, dandruff, dust bunnies — and it all sticks to your finish. If you use a spray gun, that becomes a contaminant, too, either from previous sprayed material or because most compressors have oil reservoirs; the oil gets atomized and is pumped into the hoses, and now you get oil in the air. If you use a conventional air compressor, it heats up the air, which holds moisture, and when air goes through the lines it cools down, and water will condense into the line. These contaminants will cause all sorts of problems in your finish.
For particulates, set up a good spray booth with doors and filters on it. You can also hang a sheet as a doorway; wet the sheet down first to attract the dust. Vacuum often, keep your shop clean, and wash your spray booth down after every project. Keep the shop door and windows closed if you can (see Zane’s recommendation above), or use screens if you can’t.
For clean airlines, use filters, water separators, and/or chillers to keep the air pure. Or avoid bad compressed air altogether by using an HVLP gun with a turbine air supply. This self-contained unit is essentially a glorified vacuum cleaner that blows a volume of air through a large diameter hose. There’s no oil, no condensation, no issues.
TO RECAP: For pristine finishes, install an HVAC system and spray booth, keep your shop clean, sand your wood smoothly and really well, use finishes made by the same company, take your time, read the instructions and document your steps. And before you know it, you’ll be teaching finishing classes, too.
This article originally appeared in the September 2018 issue.