Sheen control and matching

Overview

Sheen is how a surface reflects light at a grazing angle. There are two ways to control it. In a satin or flat finish the maker has stirred in a flatting agent, usually silica, which gives the surface microscopic bumps that scatter light. Alternatively, you abrade a gloss film so that scratches do the scattering. Bob Flexner's central point, tested by Wood Magazine's staff, is that only the flatting agent at the top of the final coat matters: stacking several satin coats does not make a flatter finish, and gloss under a satin coat looks no different from satin under satin.

When to use

• Matching an existing piece, or when the store only stocks gloss and satin (Flexner's example: nothing flat enough).
• Deciding between "satin on all coats" and "gloss then rub out"; see the steps below.
• Not needed on oils and hardwax oils, which are low-sheen by nature; see Hardwax oil, Tung oil.

Materials and tools

• A finish in gloss and in satin or flat, of the same make and type, or a separate flatting agent.
• Test boards sanded and prepared like the project.
• For rubbed sheens: 0000 steel wool, pumice, rottenstone; see Rubbing out a finish.

Steps

1. Decide the target and test on scrap, because you can't judge the sheen until the film has dried (Flexner).
2. Option A, off the can: apply all coats in the target sheen, or build with gloss and finish with a satin last coat. Flexner and Wood Magazine both say the last coat determines the sheen, and that satin is just gloss with flatting agent added, so hardness is the same.
3. Option B, blend: let flatting agent settle (do not shake the can, or wait a week or two), pour off the clear gloss from the top, and mix this with the flat sludge in different ratios (Flexner). This works for solvent-based finishes, not water-based, where flatting agent settles much more slowly. Many makers also sell finishes with a numeric sheen, such as 20 for fairly flat or 90 for gloss (Woodshop News), which you can blend.
4. Option C, rub: build gloss, level, then abrade. Coarse scratches (400 to 600 grit) give flat, finer ones satin, and a scratch-free film gloss (Woodcraft). Wood Magazine's answer for a cherry dining table: three coats of satin, then rub out; level to 600 grit, then 0000 steel wool, then 2F and 4F pumice in mineral oil for even satin, leaving satin on routed edges.
5. Pekovich builds gloss then rubs back for a thicker film at low luster: "build to a little higher gloss than you're aiming for, then rub it out." See Wipe-on application.
6. Thin consistently. Flatting is more effective the more you thin the finish, since it spreads thinner and more particles sit near the surface (Flexner in Woodshop News). Thin the same each time you refill the cup.

Common problems

• White specks: clumped flatting agent that has bonded at the bottom or lip of the can; change to fresh product (Woodshop News).
• Uneven sheen across a top: uneven application thickness or thinning; or a patchy rub. Redo the last coat or the rub.
• Sheen turns glossier over years: wear levels the microscopic roughness; it is not flatting agent leaving (Woodshop News).
• Shiny edges and flat top after rubbing: blend areas with a few passes over the whole surface; see Rubbing out a finish.

Safety and environment

Clean up dust after sanding; see Dust removal and tack cloths. For solvents used in wet-sanding and rubbing out, see Oily rag safety.

Related

• Build coats and sanding between: coat count.
• Spraying with HVLP: "off the gun" sheen differs from rubbed satin (Dresdner, Woodworker's Journal).
• Oil-based polyurethane, Water-based polyurethane, Pre-catalyzed and post-catalyzed lacquer: finishes sold in several sheens.