Sanding the inside of a hole comes down to matching the right tool to the hole’s diameter. Small holes under half an inch call for abrasive cords or wrapped dowels. Medium holes work well with sanding drums on a drill. Larger openings give you room for flap wheels or even oscillating spindle sanders. Here’s how to approach each size and situation.
The Simplest Method: Sandpaper on a Dowel
For most holes, wrapping sandpaper around a wooden dowel is the fastest way to get started. Choose a dowel slightly smaller than the hole’s diameter so the sandpaper has room to contact the walls without jamming. Wrap a strip of sandpaper around the dowel with the abrasive side facing out and twist it by hand inside the hole, moving up and down as you rotate. This works surprisingly well for holes roughly 3/8″ to 1″ in diameter.
You can upgrade this approach by cutting a thin slot in the end of the dowel with a handsaw. Thread a strip of sandpaper through the slot, then fold the abrasive sides outward so both layers sand as you twist. This keeps the paper from slipping. If you chuck the slotted dowel into a power drill, the spinning motion uses centrifugal force to press the sandpaper outward against the hole’s walls, turning a slow hand operation into something much faster.
Sanding Drums for Drills and Drill Presses
Rubber sanding drums are the most popular dedicated tool for this job. They consist of a rubber cylinder on a metal mandrel with a bolt that compresses the rubber, expanding it outward to grip an abrasive sleeve. Tighten the bolt and the sleeve locks in place. Loosen it and you can swap sleeves when they wear out.
Standard drum sets come in sizes like 1/2″, 3/4″, 1″, 1-1/2″, and 2″ diameters. You want the drum to be slightly smaller than the hole so it contacts the walls without binding. These drums fit into a hand drill, drill press, or lathe, giving you flexibility depending on what equipment you have. A drill press offers the most control because the workpiece stays flat on the table and you lower the spinning drum into the hole vertically.
Very Small Holes: Abrasive Cords and Rotary Tools
Holes under 3/8″ are too tight for most drums or dowels. Abrasive cord solves this problem. These are thin cords coated with grit, available as narrow as .040″ in diameter in grits like 180. You thread the cord through the hole and pull it back and forth like flossing. It’s tedious for large amounts of material removal, but for smoothing or deburring a small drilled hole, nothing else reaches.
A rotary tool like a Dremel opens up more options for small holes. Mini flap wheels, which are tiny clusters of sandpaper strips on a spindle, start at around 8mm (roughly 5/16″) in diameter. For a 12mm hole, for example, a 10mm flap wheel fits inside and expands slightly during use to about 11.5 to 12.8mm, conforming nicely to the interior surface. These mount on small shanks (2.35mm or 3.175mm) designed for rotary tool collets.
Oscillating Spindle Sanders for Larger Projects
If you’re sanding the insides of curved cutouts or larger holes regularly, an oscillating spindle sander is worth considering. These benchtop machines have a vertical sanding drum that spins while simultaneously moving up and down. That oscillation distributes wear across the full height of the abrasive sleeve, reduces heat buildup, prevents burn marks, and extends the life of each sleeve significantly compared to a drum that only spins.
Spindle diameters typically range from 1/4″ to 4″, so you can match the drum to your hole size. You guide the workpiece against the drum on a flat table surface, which gives excellent control over how much material you remove. For bandsaw box interiors, lamp holes, or any project with curved internal edges, this is the cleanest option.
Building a Custom Sanding Mandrel
When your hole doesn’t match a standard drum size, you can build your own mandrel. You’ll need a piece of threaded rod, a couple of nuts and washers, plywood discs cut to a diameter just under the hole size, and a layer of rubber or dense foam (neoprene works well). Stack the plywood discs on the threaded rod with the rubber wrapped around the outside, then tighten the nuts to compress everything together. Wrap sandpaper around the rubber layer or attach it with adhesive-backed hook-and-loop material so you can swap grits easily. Chuck the threaded rod into a drill and you have a custom-diameter sanding drum.
Choosing the Right Grit Progression
Start with the coarsest grit you actually need, which depends on how rough the hole’s interior is. A hole cut with a hole saw or Forstner bit is already fairly smooth, so you can often start at 120 or even 150 grit. A hole roughed out with a jigsaw or bandsaw might need 80 grit to knock down the tool marks first.
From your starting grit, step up gradually. A good rule: never jump more than 50% above your current grit number. Starting at 80, your next stop should be 120 at most, not 150. A practical sequence for most interior holes is 120, then 150, then 220. If you’re after a polished look on a turned piece or visible interior, you can continue to 320 or 400. For holes that won’t be seen, stopping at 150 or 180 is usually enough.
Avoiding Burn Marks Inside the Hole
Friction burns are the most common problem when power-sanding inside a hole, especially in hardwoods like cherry or maple. The enclosed space traps heat, and a spinning drum in a tight hole concentrates that heat fast.
The fix is almost always the same: slow down and lighten up. If your drill has variable speed, dial it back. Let the abrasive do the cutting rather than forcing the tool against the wood. Keep the workpiece or the tool moving constantly. Holding either one still while the drum spins creates hot spots that char the wood in seconds. Move the piece up and down along the drum, or if you’re using a handheld drill, keep the drum traveling through the hole rather than parking it in one spot.
Fresh sandpaper also matters more than you’d expect. A worn sleeve generates heat through friction but removes almost no material. If you’re pushing harder to compensate for a dull sleeve, you’re creating the perfect conditions for burning. Swap the sleeve, use less pressure, and the burn marks disappear.