Woodworking Tip of the Day: Oil Finishes: Linseed, Tung or Danish Oil, and Disposing of Oily Rags Safely

Not for nuthin'…. But….. it's Halloween, and the scariest thing in most shops tonight isn't a costume. It's a wad of oily rags balled up in the trash barrel in the corner. Before we get to that, let's straighten out what's actually in those cans of "linseed," "tung" and "Danish" oil, because the labels sure won't tell you. I've got thwarts, a tiller and a rack of tool handles waiting on the dory, so I went back to the makers' own directions and Bob Flexner's finishing work and wrote it all down.

Why it matters: An oil finish is about the easiest finish there is: wipe it on, wipe it off, and it brings the grain up warm. But it only cures right if you take off every bit of the excess, and every rag you used is a fire waiting to happen until you deal with it. NFPA counts an average of about 900 home fires a year started by oily rags.

What's really in the can

• Raw linseed oil is straight flax oil with no driers. It takes weeks to cure. Flexner says he knows of no interior use for it, and I'd leave it on the shelf for furniture and trim.
• Boiled linseed oil (BLO) isn't boiled. It's linseed oil with metallic driers, so it cures in about a day once the excess is off. Klean-Strip says to let it soak in 10 minutes, wipe off all the surface film, and figure up to 16 hours to tack-free and several days to fully cure. Good on tool handles, mallets and shop jigs.
• Pure tung oil gives you the best water resistance you'll get from a straight oil, but it's slow. Waterlox says to thin it about 1:1 for most work, let each coat soak in for up to 45 minutes, and keep re-wetting until it sits on top. Then wipe off all the excess and give it 7 to 14 days to dry and up to 30 days to fully cure. Flexner figures five to seven coats, with two to three days between them and a light sanding after every coat.
• "Danish oil" and most "tung oil finish" products are oil/varnish blends. Flexner lists Watco Danish Oil and Minwax Tung Oil Finish as blends, and some other "tung oil" products (Formby's, for one) as thinned wiping varnish. A blend gives you a little more protection than plain oil and is just as easy to wipe on. One more curveball: Tried & True calls its straight polymerized linseed oil "Danish Oil." Read the ingredients, not the name.

Where oil belongs in a boat shop: interior trim, thwarts and seats on a boat that lives under cover, tool handles, bench tops and cabinets. Oil alone isn't exterior brightwork. It darkens in the sun and needs regular maintenance coats, and Watco, Minwax Tung Oil Finish and Tried & True are all labeled for interior use. For brightwork that lives out in the salt air, oil is a maintenance plan, not a finish. (If you want a film that builds, that's the wiping varnish from the Oct. 19 tip.)

Materials

• Your oil: BLO, pure tung (plus mineral spirits or citrus solvent to thin it), or an oil/varnish blend like Watco Danish Oil
• A pile of clean, lint-free cotton rags (old T-shirts work)
• 0000 steel wool or 220-grit paper for between coats
• An old metal paint can with a tight lid, water and a squirt of dish detergent
• Nitrile gloves

Tools

• Cheap natural-bristle brush or a folded rag for flooding it on
• Random-orbit sander or a sanding block for prep
• A clear spot outdoors, away from the building, plus a few rocks or scrap blocks to weigh rags down

Steps

1. Read the can first. If mineral spirits or petroleum distillate is on the ingredient list, it isn't pure oil. If you're not sure, do Flexner's puddle test: half a teaspoon on a piece of glass, then wait 24 hours to a couple of days. Smooth and hard means wiping varnish. Soft and wrinkled means an oil/varnish blend.
2. Prep bare wood. Get every old finish off, sand smooth (Watco's sheet runs 120, 150, 180, 220), and vacuum or wipe off all the dust. Oil goes on bare wood only, because it has to soak in to cure.
3. Flood it on. Use a brush or rag and lay it on heavy, adding more wherever it soaks right in. End grain will drink it.
4. Let it soak, on the label's clock. Watco Danish Oil: 30 minutes, then wet it again and give it 15 more. Minwax Tung Oil Finish: 5 to 10 minutes. BLO: about 10 minutes. Pure tung: up to 45 minutes, re-wetting the dry spots until the oil just sits there.
5. Wipe off ALL the excess. Use clean, dry rags, with the grain, until the surface feels dry. Then change rags and do it again. Anything you leave on top cures soft and gummy. This is the step that makes or breaks an oil finish.
6. Come back for the bleed-back. Open-grained wood like oak and mahogany pushes oil back out of the pores. Check it over the next hour or so and wipe off any shiny beads. On pure tung, keep a rag handy for the first couple of weeks.
7. Let it cure. Watco says it's ready for light use in 8 to 10 hours. Minwax says 24 hours between coats. BLO, overnight at least. Pure tung takes days. Cold shops are slower, and Watco wants 60 to 90°F.
8. Scuff and recoat. Knock down the raised grain with 0000 steel wool or 220, wipe off the dust, and repeat. Two or three coats of BLO or a blend (Minwax says two), and five to seven for pure tung. Going over it with varnish? Watco says to wait 72 hours first.
9. Deal with the rags right then and there. See below. Not after supper.

The rags. Read this part twice. Linseed and tung oil cure by soaking up oxygen, and that reaction gives off heat. Spread out flat, the heat just drifts away. Wad it up in a pile or a trash bag and the heat builds until the rag lights itself, hours later, after you've shut the lights off and gone in the house. NFPA and the Massachusetts fire officials say:

• Never leave oily rags in a pile, a box or the trash.
• Option A: Hang them or spread them flat outdoors in a single layer, weighed down so they don't blow around, away from buildings, until they're completely dry and stiff. Klean-Strip says at least 24 hours above 40°F for BLO rags, and Waterlox figures 7 to 15 days for pure tung.
• Option B: Put them in a metal can with a tight lid (an old paint can is perfect), cover them completely with water and a little detergent, and keep the can somewhere cool and out of the sun. Watco's safety sheet says to put soaked rags straight into a sealed, water-filled metal container.
• Either way, the can of rags goes to your town's household hazardous waste collection, not the barrel. Check with your town for the date.
• The same goes for steel wool, paper towels, brushes and your shop apron if it got soaked.

MOAM's shop note: The mistake I see most is the "one more coat for good measure" fella who floods it on and walks away. Oil isn't varnish. It doesn't build, and whatever's left on top turns into a sticky, wrinkled mess that you'll be scraping off in a week. Wipe it like you're trying to take it all back off. The wood keeps what it needs.

Sources: Watco Danish Oil technical data sheet and Rust-Oleum safety data sheet, Minwax Tung Oil Finish directions, Klean-Strip boiled linseed oil guidance, Waterlox 100% Pure Tung Oil application guide, Tried & True product directions, Bob Flexner, "Oil Finishes: Their History and Use" (Popular Woodworking), Hendrik Varju, "The Confusion Over Wipe-On Finishes" (Fine Woodworking #207), Power & Motoryacht, "Varnishes, Oils, and a Little Heresy," NFPA "Oily Rags" safety tip sheet, and Mass.gov, "Disposing of oily rags."

MOAM Out.