Woodworking Tip of the Day: Sanding Sequence: Which Grits You Can Skip and Which You Can't

This is catching the MOAM's attention today….. how many good finishes get ruined with sandpaper. Either somebody jumped from 80 grit straight to 220 to save twenty minutes, or somebody polished cured epoxy so slick the varnish had nothing to hang on to. I'm building a Chamberlain Marblehead gunning dory, and between bare wood, epoxy-coated planks and primer, I go through a lot of paper in this shop. So today: which grits you can skip, which ones you can't, and where to stop for wood, for epoxy under varnish, and for epoxy under paint.

Why it matters: Each grit's only job is to take out the scratches the last grit left. Skip too far and the finer paper just knocks the tops off the deep scratches. Lon Schleining put it plain in Fine Woodworking: no matter how long you sand, 180-grit won't remove the scratches made by 100-grit. They hide until the stain or the first coat of varnish lights them up. Go the other way, too fine on a surface that's getting another coat, and you've sanded off the "tooth" the coating needs to grab. WEST SYSTEM, Epifanes and Interlux all warn about that one.

The rules for skipping

• Start coarse enough, not coarser. Bob Flexner (Popular Woodworking) starts planed solid stock at 80 and factory-sanded plywood at 120, because 80 on veneer is wasted work and risks sanding through. Schleining starts hard maple at 120, since 80 and 100 scratches are tough to get out of it.
• Never skip two in a row. Uneeda, the abrasives maker, calls it the golden rule: never skip more than one grit. 80, 120, 180 is fine. 80 straight to 180 isn't.
• By hand, with the grain, you can skip one. Jeff Jewitt (Fine Woodworking) says hand sanding leaves scratches that run with the grain, so you can go 100, 150, 220. The American Woodworker editors checked with sandpaper makers and got the same answer: 80, 120, 180 or 100, 150, 220.
• On a random-orbit sander, don't skip. Jewitt again: those machines put cross-grain scratches in the wood, and if you skip, you'll likely see the coarse ones later. Flexner calls every grit (80, 100, 120, 150, 180) the most efficient way to go, as long as you only sand each one long enough to take out the last.

Where to stop depends on what goes on next

• Bare wood under a film finish (varnish, shellac, lacquer): Flexner says it's rarely worth going finer than 180, because after a couple of coats the finish makes its own surface. Go to 220 if random-orbit swirls still show, and finish by hand, with the grain, at your last grit. Water-based finish is the exception; Jewitt goes to 320 or 400 to cut down the raised grain. (Raising the grain was the 10/13 tip.)
• Bare wood under marine varnish: Epifanes says bare wood should normally be prepared to about 100 or 120 grit, and that much finer won't give that first coat enough grip. Then 220 dry over the first few coats to knock down the raised grain, and 320 wet or dry once the grain fills, right to the end. 400 is the finest they'd ever use between coats. Anything finer, there's no tooth.
• Epoxy-coated brightwork under varnish: TotalBoat says let the epoxy cure 5 to 7 days, wash it with mild soap and warm water even if it's sold as "non-blushing," dry it, then sand with 220 or 320. Only reach for 80 to cut down runs and sags (WEST), then climb back up through the grits.
• Epoxy under paint: WEST SYSTEM's manual says the coating maker's directions rule, and gives the ballpark: 80 to 100 grit is usually enough under a high-build or filling primer, 120 to 180 for primers and high-solids coatings, and 180 is often recommended for high-gloss finishes. Finer than that may not give enough tooth and can lead to sags and runs. Interlux's system for a clear-epoxy surface: let it cure 3 to 7 days, wash off the blush first (Interlux says sanding doesn't remove contamination), sand 80 to 120, fill and sand 120 to 220, then a two-part primer (Epoxy Primekote), 120 between primer coats, and 220 to 320 on the last primer coat before the topcoat. Their one-part Brightside topcoat gets sanded 220 to 320 between coats.

Materials

• Sanding discs for your random-orbit sander: 80, 100, 120, 150, 180, 220
• Sheets for hand and wet sanding: 120, 180, 220, 320, 400 wet-or-dry
• Maroon Scotch-Brite pads and clean water (for washing epoxy before sanding)
• A soft pencil
• The coating's data sheet or can label, so you know your last grit before you start

Tools

• 5-inch random-orbit sander hooked to a shop vac
• A flat sanding block (cork-faced or rubber) for flat work, and a longboard for fairing a hull
• A bright light you can set low for raking across the surface
• Shop vac with a brush head, and tack cloth for between coats
• Half-mask respirator with P100 filters for epoxy, primer and fairing dust

Steps: climb the ladder without missing a rung

1. Read the can before you pick up the sander. The coating sets your last grit, so know where you're stopping before you start: 180 for most film finishes on bare wood, about 100 to 120 under Epifanes varnish, 220 to 320 on epoxy under varnish, and whatever the primer's sheet says under paint.
2. Pick the starting grit by the worst defect, not by habit. Planer marks on solid stock: 80. Factory-sanded plywood: 120. Hard maple: 120. Cured epoxy with runs or sags: 80, just on those spots. A smooth epoxy coat that only needs tooth: go right to your finish grit.
3. Epoxy gets washed before it gets sanded. Let it cure all the way, then scrub with water and a maroon Scotch-Brite pad, and dry it. Sanding first just grinds the blush in. (The full story on amine blush was the 10/24 tip.)
4. Scribble pencil across the surface. The American Woodworker editors use pencil lines to tell when glued-up boards are flush. I use them for every grit: when the scribble's gone everywhere, that grit's done its job.
5. Sand, then check with a raking light. Set the light low and look across the surface before every grit change. One stray scratch is easy to fix now. Find it after the next grit and you're backing up two.
6. Vacuum, then move up one grit. On the random-orbit sander, go every grit. By hand, with the grain, you can skip one. Never skip two. Vacuum between grits so a loose coarse grain doesn't get dragged across your fine work.
7. Finish by hand, with the grain. After your last machine grit, hand-sand with a flat block at the same grit to take out the random-orbit swirls (Flexner).
8. Test it, clean it, coat it. On epoxy, rinse with fresh water: WEST says it should sheet out evenly. If it beads, wet-sand that spot again. Then wipe with whatever cleaner the coating maker calls for and get the coat on.
9. Between coats, sand light and not too fine. Wait until it's hard enough to sand. Epifanes says one-part varnish or paint is generally safe after 24 hours. Interlux's Brightside sheet says 12 hours at 75°F, 16 at 60°F and 20 at 50°F. Then use 220 to 320, and never finer than 400.

On the dory: The epoxy-coated planking that's headed for paint and any bright trim headed for varnish get two different ladders. The trim gets washed and taken to 220 or 320 for varnish. The hull gets washed, sanded 80 to 120, and goes under a two-part primer before any one-part topcoat, because Interlux's Brightside sheet calls for Epoxy Primekote or InterProtect over clear epoxy before the one-part topcoat goes on. And Primekote and Brightside both want the air, the paint and the hull at 50°F or warmer, so in a Marblehead shop in November, that's a warm-day job.

MOAM's shop note: The most common mistake I see is the guy who sands his last coat of epoxy to 600 because it feels like glass, then wonders why the varnish peels in the spring. Smooth isn't the goal. Tooth is. Epifanes says anything finer than 400 won't give enough mechanical bond, and WEST says going finer than about 180 under paint may not give enough tooth for good adhesion. If you want it slick, do it with the finish, not the sandpaper. The other mistake is the guy who goes 80 to 220 on a random-orbit sander, puts stain on, and finds a tabletop full of little curlicues. Every grit, on the machine. It's faster than doing it twice.

The numbers here come from the makers' data sheets and help pages (WEST SYSTEM, Interlux, Epifanes, TotalBoat and Uneeda) and from Fine Woodworking, Popular Woodworking and American Woodworker. Read the label on your can. If the coating maker says something different, the coating maker wins.

Sources: WEST SYSTEM, User Manual (Final surface preparation). Interlux (AkzoNobel), Epoxy Primekote data sheet, Brightside Polyurethane data sheet and product information sheet, and the Perfection application bulletin. Epifanes NA, Q & A. TotalBoat, Varnishing over epoxy. Uneeda, Grit Sequence. Bob Flexner, Rules for Sanding Wood (Popular Woodworking). American Woodworker, 12 Tips for Faster, Smoother, Better Sanding. Fine Woodworking, Lon Schleining, Tips for Better Sanding, and Jeff Jewitt, Sanding Basics.

MOAM Out.

Tools for this tip

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• Diablo SandNet 5 in. assorted sanding discs with connection pad, 10-pack (2 each 80/120/180/220/400), DND050ASTH10I: one pack covers the climb: 80, 120, 180 and 220, plus 400 for between coats; fits 5- and 8-hole random-orbit sanders
• 3M 05519 rubber sanding block, 2-3/4 x 5 in. (wet or dry): a flat block for hand sanding with the grain and wet-sanding primer and epoxy
• 3M Pro Grade Precision sanding sheets, assorted 220/320/400: 220/320/400 sheets for sanding between coats of varnish and paint
• 3M Scotch-Brite 7447 general purpose hand pads, maroon, 6 x 9 in. (20-pack): the maroon pad for washing cured epoxy with water before you sand it
• Oneida Air Systems Dust Deputy DIY anti-static cyclone separator for wet/dry shop vacs: cyclone between the sander and the vac, so the fine dust lands in the bucket
• 3M 6503QL Rugged Comfort Quick Latch half facepiece reusable respirator, large: half-mask respirator for sanding epoxy, primer and fairing compound

Use whatever you already have in the shop; these are just the ones that fit this job.