4" x 21" Sanding Belts, 9 PACK
4" x 21" Sanding Belts, 9 PACK
Available in stock (49997)
A 4" wide portable belt sander covers a third more surface per pass than a 3" machine, which is why the 4" x 21" size shows up on the sanders used for doors, countertops, wide planks and floor edges where a 3" belt leaves too many overlapping tracks. The extra width also spreads the load, so the belt runs cooler on a long pass.
We convert these belts on a poly-cotton cloth backing with heat-treated grain in Anaheim, California. Cloth A/O is the wood, paint and finishing belt and the only grain stocked from 150 through 600; Zirconia takes on hardwood levelling and mild steel; Ceramic is for hardened and high-alloy steel under firm pressure. Nine belts per pack, 24 through 600 grit.
- Cut for 4 x 21 portable belt sanders; measure your belt or check the manual before ordering
- Poly-cotton cloth backing with heat-treated grain outlasts the paper belts sold with the tool
- Cloth A/O, Zirconia and Ceramic in a single size
- 24 to 600 grit covers stripping, levelling, pre-finish and fine finishing
- 9 belts per pack; free shipping on orders over $200
- Any other 4" length made to order in about 3 days
Which 4" x 21" belt material should you choose?
The grain has to suit the pressure a hand-held sander can apply and the material under it.
- Cloth A/O (aluminum oxide): the everyday belt for a 4" sander. It levels doors and countertops, strips paint and varnish, smooths plywood and softwood without tearing, and is the only grain we stock in this size above 120 grit. The wider belt lets Cloth A/O work at the low pressure it likes while still removing material quickly because more grain is in contact.
- Zirconia: for fast removal on dense hardwood, laminated beams and butcher block, and for mild steel and weld beads. Zirconia fractures to expose fresh edges only under load, so lean into the sander on flat stock; keep it off veneer, softwood and painted trim. Stocked 24 through 120 grit.
- Ceramic: a self-sharpening grain for hardened steel, stainless and alloy plate. On a 4" portable sander it earns its cost only when you bear down steadily on flat steel; under a light touch it glazes and puts heat into the part. Metal fabrication and repair shops buy it; woodworkers should choose Cloth A/O.
The previous version of this page also listed silicon carbide. It is not stocked in 4" x 21" but can be quoted as a custom belt. Not every grain is stocked in every grit; the variant selector shows current combinations.
Grit guide for wide-surface sanding
| Grit | Use it for | Notes |
|---|---|---|
| 24–40 | Heavy paint removal, rough lumber, steel welds | Zirconia or ceramic; not on veneer |
| 50–60 | Levelling doors, countertops, glue lines | Usual first grit on solid wood |
| 80–100 | General surfacing, removing the coarse scratch | Most of the work happens here |
| 120–150 | Pre-finish; plywood and veneer | Start here on veneer |
| 180–220 | Final belt before stain or clear coat | Cloth A/O, with the grain |
| 320–600 | Between-coat scuffing, fine finishing, non-ferrous pre-polish | Light pressure, keep moving |
Never jump more than one grit. A 4" belt hides the coarse scratch under a broad, even-looking surface until the finish goes on.
Which sanders take a 4" x 21" belt?
- 4" wide portable belt sanders whose manual specifies a 4" x 21" belt (some manufacturers list it as 100 x 533 mm). The size is less common than 4" x 24" and 3" x 21", so confirm before you order.
- How to check without the manual: remove the old belt, cut it at the joint and measure end to end. A 21" belt measures 21" (533 mm); a 24" belt measures 24" (610 mm). A 4" x 24" belt will not tension on a 4" x 21" sander and a 21" belt will not fit a 24" machine.
- Bench or stand-mounted 4" x 21" machines and inverted portable sanders clamped in a stand for small-parts work.
Mounting: retract the front roller with the tension lever, slide the belt on so any direction arrow on the inside points the way the belt travels, lock the lever, then run the sander upside down and turn the tracking knob until the belt sits centred on the platen without touching the housing. Check the inside of the belt for a direction arrow before mounting.
How to get the most out of it
- Start the sander off the work and land it moving. A 4" belt removes wood fast; a stationary start leaves a dish the width of the belt.
- Overlap passes by half the belt width and work the full length of the board each time. Check flatness with a straightedge as you go on doors and countertops.
- Let the sander's weight set the pressure. A 4" machine is heavy enough; pushing loads Cloth A/O and burns maple and cherry.
- On glue-ups, level at a slight diagonal with the first grit, then sand straight with the grain at the next grit to remove the diagonal scratch.
- On veneer, start at 120 grit Cloth A/O, make one or two light passes and stop; face veneer is thin.
- Use the wide belt for edges too. Clamp the door or panel on edge and keep the platen square to the face; the 4" width bridges the edge and resists rocking.
- Empty the dust bag and clean the belt. Press a crepe rubber belt eraser against the running belt when paint or resin packs the grain.
- Metal: zirconia or ceramic with firm pressure on steel; Cloth A/O with a grease stick on aluminum. Keep sparks away from wood dust.
Use it for / don't use it for
Use it for
- Levelling and refinishing doors, countertops, workbench tops and wide planks
- Edge-sanding floors where the drum sander cannot reach, and stair treads
- Stripping paint, varnish and weathered surface from solid wood
- Flattening glued-up panels and butcher block
- Dressing welds, mill scale and plate (zirconia, ceramic)
- Between-coat scuffing over large flat areas with 320 through 600 grit Cloth A/O
Don't use it for
- Narrow stiles, rails and trim where a 4" belt overhangs and rounds edges
- Zirconia or ceramic on veneer, softwood or painted trim
- Cross-grain passes on anything that will be stained
- A sander specified for 4" x 24"; the belt will not tension
Specifications
| Size | 4" wide x 21" long |
|---|---|
| Abrasive grains | Cloth A/O (aluminum oxide), Zirconia, Ceramic |
| Backing | Poly-cotton cloth, heat-treated grain |
| Coat | Closed coat |
| Joint | Check the inside of the belt for a direction arrow before mounting |
| Grits offered | 24, 36, 40, 50, 60, 80, 100, 120, 150, 180, 220, 320, 400, 600 (zirconia 24–120) |
| Pack quantity | 9 belts |
| Wet or dry | Cloth backing tolerates both; run dry on electric portable sanders |
| Converted in | Anaheim, California |
| Custom sizes | Any width and length made to order in about 3 days |
Frequently asked questions
How do I know whether my sander takes 4" x 21" or 4" x 24"?
Check the manual or the label on the belt cover first. If neither is available, cut the old belt at the joint and measure it flat: a 4" x 21" belt is 21" (533 mm) long and a 4" x 24" belt is 24" (610 mm). The two are not interchangeable; a 24" belt will not tension on a 21" sander.
Which grit should I use to level a solid wood door?
Start at 60 grit Cloth A/O if the door has raised joints or old finish, or 80 if it is fairly flat. Work with the grain in overlapping full-length passes, then move to 100 or 120 and finish at 150 or 180 before stain. Use a straightedge across the width after each grit.
Is a 4" x 21" zirconia belt worth it for woodworking?
Only for heavy levelling of dense hardwood, laminated beams or butcher block in 24 through 60 grit, where you can lean on the sander and the wood can take an aggressive cut. For general surfacing, softwood, plywood and any grit above 120, Cloth A/O gives a cleaner result and costs less.
Can I use these belts on aluminum or steel?
Yes. For mild steel, weld beads and mill scale use Zirconia in 24 through 80 grit with firm pressure. For hardened steel and stainless use Ceramic and keep steady pressure so the grain stays sharp. For aluminum use Cloth A/O with a grease stick to prevent loading. Finish above 120 grit with Cloth A/O and keep sparks away from sawdust.
Why does my belt keep loading up with paint?
Paint and finish soften with heat and pack into the grain when the sander dwells or is pushed. Use a fresh 40 to 60 grit Cloth A/O belt, keep the sander moving, do not press, and clean the belt every few passes with a crepe rubber belt eraser. Very thick or gummy coatings are better scraped or stripped first.
Why nine belts in a pack?
A 4" x 21" pack of nine gives you three belts each in three grits for a typical door or countertop job, or nine of one grit for production use. Belts are sold one material and one grit per pack; for a mixed pack or case pricing, call 714-646-3299 or email sales@maverickabrasives.com.
What if my machine needs a slightly different 4" length?
We make any 4" wide belt length to order in about 3 days, in the same Cloth A/O, zirconia and ceramic grains. Send the exact belt length from your manual, or measure a cut belt end to end, and we will quote it. Unused stock belts in original packaging can be returned within 30 days.
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