slab bench

↻ 2026-05-25

• overview
  • slab dimensions
• finishing
  • working window
  • sanding
  • water popping (optional)
  • dust removal
  • mixing
  • application
  • drying and cure
  • oily rag disposal
• base
  • design
  • cuts and welds
  • finishing the steel
  • assembly

overview

I’m making a bench from one of the white oak slabs I chainsaw-milled and later kiln-dried. The base design is TBD. This page tracks the finishing for now, since the slab is in hand and I have a stretch of decent weather coming.

I’m working in the carport, not a temp/humidity-controlled shop, so the finish plan has to fit that constraint. I’ll be picking my day for application instead of setting up an environment.

slab dimensions

finishing

I’m using Rubio Monocoat Oil Plus 2C in Dark Oak. The product is two parts that get mixed at application: Oil Plus 2C Component A (the colored oil), and Accelerator Component B (the catalyst). It’s a 0% VOC, single-coat hardwax oil that bonds chemically to bare wood. One layer, no buildup, no overlap marks, food-safe once cured. White oak takes it well, and Dark Oak gives the slab a deeper aged-oak tone without going black.

A 350 ml A+B set is plenty for this slab. The set covers 105 to 175 ft², and I have about 17.

working window

Rubio is fussy about the cure environment, and the chemistry isn’t forgiving of the wrong values. Since the carport is open to the weather, I’m picking a day (and a 5-day window after it) that lands in range, instead of running a thermostat.

Some carport-specific notes:

• Pick a day with steady temperatures, including overnight. If the low drops below 60°F during the first 48 hours, the cure stalls.
• No rain in the 5-day forecast. A carport keeps direct rain off the slab, but blowing rain and splashback will still hit it, and the ambient humidity around a storm sits well above the 60% ceiling.
• No pollen storms or windy dust either. The slab is wet and tacky during the 30-minute wipe window, and anything that lands on it gets locked in.
• The slab stays in the shade. Direct sun will heat the surface above the 77°F ceiling and shorten the 4-to-6-hour pot life.
• The kiln-dried MC was in range when I picked the slab up. If the slab has been sitting in the carport for a while it may have crept up. I’ll spot-check with a pin meter before I start.

If the forecast doesn’t cooperate, I wait. There’s no rush.

sanding

Random-orbital sander with hook-and-loop discs. Final grit is 150. Rubio is specific that you do not go finer than that. Anything higher burnishes the surface, closes the pores, and the oil can’t get in, which kills both the color and the durability. So if you’re used to running a build up to 220 or 320 for a film finish, don’t do it here.

Progression on both faces, both end grains, and all four edges:

1. 80 grit to knock down planer marks and any cup, until it reads flat against a straightedge.
2. 100 grit to remove the 80-grit scratch pattern.
3. 120 grit to refine.
4. 150 grit. Stop here.

Between grits, vacuum the slab and the floor, then wipe with a dry rag. A piece of 80 grit caught under a 150 disc leaves a single deep scratch that the oil will outline. I’ll check each step under a raking light at the edge of the slab and re-sand any swirls or remaining mill marks before stepping up.

End grain gets the same progression. It’ll drink more oil than the faces, which is normal for end grain.

water popping (optional)

For a slightly deeper Dark Oak, I’d wipe the 150-sanded surface with a damp (not wet) rag. The grain raises as it dries, 30 to 60 minutes, then a quick hand pass with 150 knocks back the fuzz. I’ll decide after seeing a test color on an offcut.

dust removal

No tack cloths. The wax in them messes with the Rubio bond. Rubio specifies their own Raw Wood Cleaner, which I don’t have, and the official Rubio sub is acetone. Mineral spirits is the obvious-seeming choice and the wrong one - Rubio explicitly calls it out because it leaves an oily residue that defeats the molecular bond.

Pure acetone is the right answer. I should have some in the shop for cleaning brushes, and if not, it’s a $10 quart at any hardware store. Nail polish remover is NOT a substitute, the cosmetic versions have glycerin, fragrance, and conditioners that leave residue.

Process:

1. Vacuum everything, including the underside and the ends, with a brush attachment.
2. Wet a clean lint-free white cotton rag (an old undershirt works) with acetone - damp, not dripping.
3. Wipe the slab in long single-direction passes, flipping or replacing the rag once it darkens. Both faces, all edges, end grain.
4. Let it flash off. Acetone evaporates fast, the surface is dry in about 5 to 10 minutes.

Nitrile gloves only - acetone dissolves latex and a lot of other glove materials. Don’t smoke or run a torch nearby, the carport ventilation is fine but acetone vapor is very flammable.

mixing

The 350 ml set is two cans: a larger one of Dark Oak Part A, and a smaller one of Accelerator Part B.

1. Stir Part A thoroughly until the color is uniform through the can. Dark Oak’s pigment settles, so this matters.
2. Add Part B to Part A at a 3:1 ratio by volume (3 parts A, 1 part B). For the full 350 ml set, that’s everything in both cans poured together.
3. Mix for 1 to 2 minutes.
4. Pot life is 4 to 6 hours from this point. Don’t mix more than you’ll use in that window.

I’ll do a small test on a sanded offcut from the same slab before committing. Rubio’s color chart is shot on white oak but they explicitly say it’s reference only, and the actual color depends on this specific slab’s grain and whether I water-popped it.

application

I’ll work in zones the size of one slab face (about 7 ft²): one zone for the top, one for the bottom, edges last.

1. Spread a small amount of mixed oil with a plastic spreader or small squeegee.
2. Work it into the wood with a Rubio applicator pad on a sanding block.
3. Let it sit for at least 5 minutes so the bond can form.
4. Within 30 minutes of first contact, wipe all the excess off with a clean terry cloth in firm overlapping passes. The surface should feel dry to the touch when I’m done.
5. Still inside that 30-minute window, do a second pass with a fresh terry cloth to lift anything left.

If excess sits past 30 minutes it cures into sticky or shiny patches, and the only fix is to sand it back to bare wood and redo. White oak isn’t on Rubio’s high-absorbency list (walnut, cherry, pine, cedar, redwood), so the standard 5-minute soak is enough.

Nitrile gloves the whole time. It’s 0 VOC but an hour of skin contact still isn’t great.

drying and cure

Through the cure, the slab stays in shade, out of any direct rain or splash, and I don’t stack anything on it. I’ll drape a light cotton sheet over it raised on a couple of blocks during cure - keeps pollen, leaves, and bugs off the surface without sealing in moisture the way a tarp would.

oily rag disposal

Terry cloths soaked with Oil Plus 2C will spontaneously combust if I leave them in a pile. Acetone rags from the dust-removal step are also flammable, but they flash off and aren’t combustion-prone in the same way.

Everyone seems to have various disposal methods involving the trash, but this seems cumbersome, and I put them in the fire pit and burn them.

base

The base is two trapezoidal end pieces connected by a single low stretcher, unpainted. The whole thing is built from rectangular tube steel, 1.5“ x 0.75“, 0.125“ wall. After welding it gets ground clean, brought to a uniform satin with scotch-brite, degreased, and clear-coated so the bright steel stays bright.

The slab is 1720 mm long and the bench seats two adults comfortably. I’m targeting a finished height of 18“ (457 mm) at the top of the slab, which is on the tall side for a bench and right for slipping up to a 30“ table. Slab is 48 mm thick, so the steel frame stands 409 mm.

design

The trapezoids splay in the short axis of the bench (looking at the bench from the end, you see an A shape). The wider footprint at the floor handles side-to-side lean without any cross-bracing. Long-axis stability comes from the slab itself plus the single stretcher running between the trapezoid bottoms.

The stretcher rides low, tucked along the floor, so it doesn’t get in the way of feet sliding under the bench when someone sits down.

Slab attaches via four steel tabs, two welded to the inside face of each top tube. Tabs project up against the underside of the slab. M8 bolts pass up through the tabs and thread into M8 inserts set into the slab. The holes in the tabs are slotted across the slab’s grain to allow seasonal movement: the inserts are fixed in the wood, but the slots let the slab shift over the bolts as it expands and contracts. White oak across a 388 mm wide slab can move 3 to 5 mm with the season - if I bolt it rigid in both axes, the slab cracks or the inserts tear out.

I’m using steel knife-thread inserts (the kind with aggressive external wood threads and an internal hex drive), not brass press-ins. Brass strips out of oak under load. The inserts seat flush in the underside of the slab and are reversible: I can lift the base off any time.

cuts and welds

Cuts are all done on a chop saw with a metal-cutting blade.

• Top tubes: 280 mm long, ends cut square.
• Bottom tubes: 360 mm long, ends cut square.
• Legs: cut at the splay angle (~5.6°) at both ends, so the leg sits flush against the top and bottom tubes when assembled. Mitered corners would look cleaner from the side but they’re harder to weld up tight and the flat-against-flat joint is more forgiving.
• Stretcher: 1220 mm, ends cut square, weld to the inside face of the trapezoid bottom tubes.
• Tabs: cut from 1/8“ flat bar, ~60 mm x 40 mm, one per attachment point (four total). Each tab gets a single M8 clearance slot, drilled 9 mm round then filed out to a ~14 mm long slot. The slot runs across the slab’s grain direction once the tab is welded in place.

I tack-weld both trapezoids on a flat surface (the concrete slab of the carport works, with the tubes shimmed level). Once everything is square, I run full welds at each joint. MIG is what I have - the welds are heavier than TIG but they grind out fine. After welding, I check the trapezoids are still flat and not pulled by the heat.

Threaded inserts for the leveling feet: drill 9 mm holes in the underside of the bottom tubes at each corner, weld M8 nuts inside, then thread in M8 leveling feet with plastic bases. The plastic keeps the steel off the concrete and lets me dial out any rocking.

finishing the steel

The goal is a bright satin finish that stays bright. Raw mild steel flash-rusts fast in humid air, especially in a carport, so the final sand and the first poly coat have to happen in the same session - within an hour or two, not the next morning.

1. Grind all welds flush with a flap disc.
2. Strip mill scale from every surface. 60-grit flap disc to start, then 120.
3. Final sand with maroon scotch-brite (roughly 320-grit equivalent), pulled in long single-direction strokes along the length of each tube so the grain reads consistent.
4. Wipe all surfaces down with acetone on a lint-free rag. Same drill as the slab dust-removal step. Nitrile gloves on, and they stay on from this point.
5. Immediately spray three thin coats of clear oil-based polyurethane. I’m using Rust-Oleum Triple Thick Polyurethane in clear gloss from the can.
6. Light scuff with 320 grit between coats once each coat is dry to the touch (about 2 hours).
7. After the third coat, let it cure 24 hours before assembling against the slab.

Oil-based polyurethane has a slight amber tint that warms the steel a touch. With the Dark Oak slab on top, the two finishes read together. Water-based clear stays neutral but is less durable for something that gets scuffed and sat on, so I’m going oil-based.

A note on flash rust: between scotch-brite and acetone wipe, I don’t touch the steel with bare hands. Fingerprints leave salts and oils that bloom under the clear coat within a week. Gloves the whole time, and if I have to set the frame down between steps, it goes on a clean piece of cardboard, not the concrete.

assembly

The frame and the slab get joined after both finishes are fully cured (5 days for the Rubio, 24 hours for the poly).

1. Flip the slab upside down on a moving blanket.
2. Set the frame on the underside, positioned with 250 mm of slab overhang at each end and centered side-to-side.
3. Mark the insert locations through the center of each tab slot with a fine pencil or scribe.
4. Lift the frame off. At each marked point, drill the pilot hole spec’d for the M8 insert (typically around 10 mm for hardwood knife-thread inserts, but follow the insert manufacturer’s chart). Depth set with a tape flag on the bit so I don’t punch through the 48 mm slab. Aim for ~30 mm depth.
5. Drive the inserts in with the recommended tool (usually an internal hex on a long allen key, or an insert driver in a drill press). Hardwood needs steady downward pressure to start the threads. The insert seats flush with the slab’s underside.
6. Set the frame back in position. Drop an M8 socket head cap screw through each tab slot (with a flat washer under the head) and thread it into the insert by hand. Bolts ~25 mm long, so they bottom in the insert with plenty of thread engagement and don’t poke through to the top of the slab.
7. Snug each bolt with an allen key. Not gorilla-tight - the slot needs to be able to slide under the washer as the slab moves seasonally.
8. Flip the bench upright, level it with the M8 feet on whatever floor it lands on.

If I ever need to take the bench apart for moving, it’s four bolts and it comes apart clean.

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