# Surface Preparation for Glue

Hewn Shop Notes · https://hewn.build/shop-notes/surface-preparation-for-glue

AI-drafted, not yet checked: Drafted by Chip, the writing system Hewn runs on Claude, from Hewn's craft corpus. Jonathan Salomons has not read this note yet, and no outside reader has checked it. Read it as a shop note: check a number before you cut to it.

Cite as: "Surface Preparation for Glue." Hewn Shop Notes, hewn.build/shop-notes/surface-preparation-for-glue. Drafted 2026-05-03; AI-drafted, not yet checked.

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Here's the thing about failed glue joints: most of them were failed before the glue went on.

The adhesive does exactly what it's supposed to do. It cures against whatever surface you gave it. If the surface was oxidized, wet, oily, or sprung, the joint fails. The glue isn't to blame.

Four things make a surface ready. All four have to be right.

## Machining freshness

PVA cures by mechanical penetration — the adhesive film migrates into open cells at the cut face and locks in. This requires open cells. A freshly machined surface has them. A surface that's been sitting in the shop for two or three days may not.

Surface oxidation forms a thin barrier layer at the cut face. You can't see it. The wood looks the same. Adhesion drops.

Glue within 24 hours of machining. For hide glue, 2–4 hours. A mortise cut Tuesday and glued Saturday needs to be re-machined.

If a joint has been waiting and you're not sure, take one light pass on the glue faces. A shoulder plane stroke on tenon cheeks. A plane pass on flat faces. This costs almost nothing and reopens the cells.

The BC Interior is dry. Dry shops close the oxidation window faster. Don't give it extra time and expect it to be fine.

## Moisture content

PVA is water-based. It cures by pushing water into the wood. The wood has to have room to accept it, which means the pieces need to be at an appropriate moisture content and, more importantly, close to the same moisture content as each other.

For interior furniture, 6–9% MC is the target. This matches the equilibrium MC of most conditioned spaces.

The mismatch problem is worse than the absolute number. Gluing a piece at 6% to a piece at 14% gives you a joint that's fine in March and splits in August as the wet piece moves toward equilibrium. The glue held. The wood moved. Same result.

A moisture meter costs $30 at the low end. Check both faces before any critical glue-up. Freshly milled stock needs 1–2 weeks in the shop to acclimate. Lumber that's been in a wet yard needs the same treatment even if it was kiln-dried when it left the mill.

One more thing: PVA introduces a small amount of water into the joint surface. On thin pieces or very dry wood, this raises the grain slightly at the glue line. You'll need to true the face after cure. Don't finish a face-glued panel and call it done — run a scraper across it first.

## Contamination

Contamination puts a barrier between glue and wood. The most common culprits:

**Oil finishes.** A surface wiped with tung oil or Danish oil carries that oil even after it appears dry. If a component was pre-finished and finish migrated past the masked area, treat the glue zone as contaminated. Cut it back.

**Wax.** Paste wax, carnauba, floor wax, or wax paper used as a glue-barrier on assembly fixtures — nearly invisible in thin films. If a surface has been anywhere near wax, treat it as contaminated. Acetone or naphtha wipe, then re-machine.

**Silicone.** Silicone spray, silicone polish, silicone releasing compounds. These bond to the wood surface at a molecular level. No solvent reliably removes silicone from wood. The fix is to plane or saw back to clean material. This is the reason aerosol lubricants have no place in a furniture shop.

**Machine oil.** Jointer and planer beds get oiled. The workpiece picks up a thin film. Wipe down machine surfaces before running anything that's headed for glue.

## The dry-fit

The dry-fit is not optional and is not a clamp rehearsal.

A complete dry-fit — every joint closed, every clamp set to working tension, diagonals measured — tells you four things: whether the joints actually close under load, whether the assembly sequence works, what the clamps need, and whether the assembly is square.

The joint that looks like it fits in your hands may not close under clamping pressure. The tenon that's slightly fat shows up as a joint that racks or gaps in the middle. These are fixable before glue goes on. They're not fixable after.

Set the parts in assembly order before you break down the dry-fit. When glue is on the surfaces and the clock is running, you execute the sequence you rehearsed. You don't make decisions.

## Before glue goes on

Re-machine if more than 24 hours since cutting. Check MC on both pieces. Inspect for contamination. Run the dry-fit to completion. Set out every clamp, caul, and glue tool in the order you'll need them.

Then apply adhesive.

The glue-up is won in preparation. Everything above is the winning.