Open-Pore and Closed-Pore Timber: Why the Same Finish Looks Wrong on Both

What a pore actually is

Hardwoods have vessels: columns of cells that carried water up the tree, stacked end to end and cut open when the timber is sawn. In some species those vessels are wide enough to see with the naked eye. In others they are small enough to be invisible at arm's length.

Pore size is not the same as grain figure or hardness. Rosewood and maple are both hard hardwoods. Rosewood has pores you can see across a room; maple's are too small to find with a 10x loupe on a planed surface.

That single anatomical fact drives a set of practical consequences that catch people out constantly.

The species list that matters in practice

<strong>Open-pored, large.</strong> Oak, ash, elm, walnut, mahogany, teak, iroko, sapele. On a planed oak surface the pores read as short dark dashes and it is those dashes that give oak its character. Fill them and you get a surface that looks like plastic with a wood pattern printed on it.

<strong>Open-pored, small.</strong> Cherry, birch, beech, walnut on some boards. Pores visible but fine. These will take a film finish without filling if the finish is thin, and they look crowded if it is thick.

<strong>Closed-pored.</strong> Maple, boxwood, sycamore, and most softwoods including pine, spruce and fir. Nothing to fill. On these timbers grain filler is not just unnecessary, it is damaging, because smearing filler into a surface with no pores in it leaves a residue that sits on top and goes grey.

Bench note: If a piece finished in high gloss looks like a mirror with dark spots in it, the spots are unfilled pores. If a piece looks like it has grey freckles, someone used grain filler on a closed-pore timber.

The wet test, and what it tells you

Alcohol on a rag, wiped across a planed surface. It flashes off in seconds and it simulates a finish exactly: the pores go dark, the surrounding wood goes slightly darker, and what you see for those few seconds is what you will get.

Count the pores in a 25 mm square. Under five visible pores, the timber is effectively closed and you should not fill. Between five and thirty, it is open enough that gloss will show them and you have a decision to make. Over thirty, you are going to see every one of them under a gloss and filling is the only way to a flat surface.

The second half of the test is the finish you have planned. Take three offcuts of the same timber and apply: one coat of wiping varnish, three coats of wiping varnish, and shellac. Then look at all three from a metre away and from 200 mm. What matters is not which is flattest, it is which one you actually want to look at.

The consequences that catch people out

<strong>A film finish over unfilled oak will never look like a piano.</strong> It will look like oiled oak with a hard skin on it. If that is not the intention, fill or use an oil.

<strong>Grain filler on maple produces grey freckles.</strong> Filler needs somewhere to go; with nowhere to go it stays in the surface and it is slightly grey by nature, because most fillers are silica-based.

<strong>Open-pored timber moves more than closed-pored timber, on the whole.</strong> More voids means more capacity to take on and release moisture. A filled oak top is more dimensionally stable than an unfilled one, which is one of the reasons factory furniture was always filled.

<strong>The same finish looks different on the same species, sanded differently.</strong> Sanding to 220 leaves the pores open and crisp. Sanding to 400 burnishes the surface and closes the pore mouths, and then the finish sits on top instead of soaking in. That is why 220 is the usual stopping point, regardless of the timber.

Comparison: one satin wiping varnish, four coats, on four timbers

• Oak, unfilled: pores read as texture; the finish looks thin and honest. Reads well from a metre, coarse from 200 mm.
• Oak, filled once: flat, even, the grain figure reads strongly. 35 minutes extra work.
• Maple, unfilled: dead flat, no pores visible, looks like a factory finish.
• Maple, filled: identical to unfilled, plus 40 minutes of work and a faint grey cast in raking light.