Sanding schedule

Overview

Sanding does two jobs: it removes the flaws left by milling (mill marks, tear-out, glue), and it leaves a scratch pattern fine enough that a finish will hide it. Bob Flexner's rule of thumb in Popular Woodworking is to start with the grit that removes the flaw efficiently and then work through a few steps to a final grit, not to chase ever finer paper. The final grit matters most when you stain, because the scratch pattern shows through the colour.

When to use

Always, before any finish, unless you are working only with a plane or scraper (Flexner: "unless you are solely planing or scraping, sanding is necessary"; see Scraping and planing for finish). Skip the fine grits when the finish is a clear film on a hard, close-grained wood. Do not sand pre-finished or already-sealed surfaces this way; see Finishing sheet goods (plywood, MDF, MDO) and Finishing veneers.

Materials and tools

• A random-orbit sander, plus a flat sanding block (cork or soft-faced) for hand work. Flexner notes that hand-sanding without a firm backing can hollow the earlywood, so use a flat block on flat surfaces.
• Paper in 80, 100, 120, 150, 180 and 220 grit; tear sheets into thirds to fit a block.
• A vacuum, a raking light and mineral spirits or denatured alcohol for checking.

Steps

1. Start where the flaw is. Flexner suggests #80 to #120 on newly machined solid wood, and #120 to #150 on pre-sanded veneered plywood or MDF, where #80 risks sanding through the face veneer. Cabinet-grade work in Popular Woodworking starts at #80 or #100; Southwick's plywood schedule starts at 100 grit on a random-orbit sander.
2. Step up through the grits. Each grit exists to remove the scratches of the one before; the whole sequence is usually only three or four papers (for example 100, 150, 180 or 220).
3. Finish at #150 to #180 for most work, or #220 if you stain or use a random-orbit sander that leaves swirls. Flexner calls sanding finer than #180 or #220 wasted effort except for staining or random-orbit swirls. Southwick's chart of minimum final grit under a clear varnish runs from 120 for ash, red oak and white oak, to 150 for walnut, mahogany, poplar, birch and white pine, and 180 for cherry and hard maple; sand more carefully if you stain.
4. Light touch on the random-orbit. Let the sander do the work, then hand-sand out the swirls.
5. Always make the last pass by hand, with the grain, using the final grit on a block.
6. Vacuum between grits. Flexner says dust does not need to be dug out of the pores, but loose abrasive grit must not be carried to the next grit. See Dust removal and tack cloths.
7. Check before you finish. Look under low-angle light, or wet the surface with mineral spirits or alcohol to show what a finish will show. Avoid mineral spirits when the finish will be water-based; see Raising the grain for what water does to a freshly sanded surface.
8. Between coats of oil finish, Flexner says oil finishes can be made ultimately smooth by sanding between cured coats, or sanding a coat while it is still wet, with #400 or #600 paper. That is far easier than sanding the bare wood to #400 or #600.

Species notes from the same sources: on pine, level with 100 grit and go up to 220, change paper often because pitch gums it, and use a block across the growth rings (Eastern white pine, Knotty pine). For figured wood, Southwick sands to 120 and then up to 320 when the finish is a penetrating oil (Figured wood).

Common problems

• Stain looks pale. Finer scratches hold less stain after you wipe off the excess. Flexner's simplest explanation is just that, and he rejects the idea that fine paper "closes" the pores. General Finishes, a stain manufacturer, says the opposite: that over-sanding "will close and seal the wood grain". The sources disagree; test on scrap, and stop at 180 to 220 either way.
• Stain is dark and blotchy. Sanding finer does not cure blotching. Use Pre-stain conditioner and washcoat or a washcoat, and see Finishing resinous and blotchy woods.
• Swirls show after staining. Random-orbit scratches: hand-sand with the grain.
• Pitch gums paper. Change it often (Southwick, on pine).
• Dished earlywood. Unbacked hand-sanding; use a flat block (Flexner).

Safety and environment

Wood dust is a respiratory hazard; use extraction and a respirator. Sanding residue from oil-based stains and finishes can combust spontaneously: see Oily rag safety. Compressed air blows dust around and Flexner says to use it only where there is a spray booth or exhaust.

Related

• Scraping and planing for finish for a cut surface instead
• Raising the grain, Dust removal and tack cloths for the steps either side of sanding
• Pre-stain conditioner and washcoat, Grain filler (pore filler), Sanding sealer (and vinyl sealer) for what comes after
• Repairing defects before finish, Glue squeeze-out and common finish failures
• Rubbing out a finish for sanding the finish itself