Flap Disc Picker

Grit: start only as coarse as the job needs

The lower the grit number, the faster the disc cuts and the deeper the scratch it leaves. Whitby Abrasives gives the working bands for coated flap discs:

Grit	Job
36 to 40	Heavy stock removal, chamfering, heavy bevels
40 to 60	Weld grinding and blending
60	Deburring, deflashing
60 to 80	Rust removal, lighter blending
80 to 120	Cleaning, refining, finish prep

Garage Welding puts it in one line: 36 or 40 grit for heavy weld and stock removal, 60 for general weld cleanup and beveling, 80 for blending and 120 for fine surface preparation. Its practical progression is 40, 60, 80, 120: "You do not always need every step, but a large jump can leave deep scratches that the finer disc removes slowly." The picker climbs that ladder from where the job starts to where the finish needs it to stop: 80 when flush and blended is enough, 120 before paint or powder coat. Standard coated flap discs commonly stop near 120, and for anything finer Garage Welding and Dome Abrasives point to non-woven and other finishing products, which Dome puts at 150 to 240 for pre-polishing. The full chart, with who says what about each band, is in the flap disc grit chart.

Type 27 or Type 29

A Type 27 disc is flat and a Type 29 is conical, its flaps angled down. Empire Abrasives holds a Type 29 at about 15 to 25 degrees to the work, where it puts the pressure on a small patch and blends fast, and calls it "particularly effective on contours and fillet welds." A Type 27 is held at 0 to 15 degrees (Whitby says 5 to 15) for a broad contact patch and a smoother, more controlled finish on flat surfaces. Empire's weld blend is the two together: "a 40-grit T29 for the main blending, followed by a 60- or 80-grit T27 to erase the scratch pattern." So the picker gives a Type 29 for the first pass of heavy removal or weld grinding and for every step on edges and fillets, and a Type 27 for the rest of the flat work. Whitby warns that holding a flat disc too steeply wears one edge of the flaps, and laying a conical one flat on a panel digs gouges. See also Type 27 vs Type 29 flap discs.

Grain: aluminum oxide, zirconia or ceramic

Whitby ranks the grains in three tiers. Aluminum oxide is the cheapest and the general-purpose default on carbon steel and cast iron, but it does not sharpen itself and dulls as it works. Zirconia sharpens itself under heat and pressure and suits aggressive removal on carbon steel, cast iron and stainless. Ceramic fractures as it grinds for the coolest, fastest cut and longest life, on steel, stainless, titanium, Inconel and, top-coated against loading, aluminum. Empire is blunt about weld blending: "Standard aluminum oxide discs will just burn up and glaze over under the pressure of weld blending," so it uses ceramic or zirconia there.

The catch is pressure. In Whitby's words, zirconia and ceramic "are pressure-activated: on a low-power tool or with a light touch they glaze and underperform a cheap aluminum oxide disc," so "spec the premium grain only where the tool and operator can push hard enough to fracture it; otherwise drop a rung." Tell the picker your grinder is cordless or low powered and it drops one grain. On stainless, Dome splits by grade: zirconia is the cost-effective choice for 304 at moderate pressure, and ceramic is preferred for 316, whose molybdenum makes chips stick to the grain and concentrates heat. Zirconia vs ceramic flap discs goes further.

Stainless, aluminum and cast iron

• Stainless. The grits are the same as for carbon steel; the disc is not. Empire and Dome both call for INOX discs, under 0.1% iron, sulfur and chlorine, kept for stainless only, because iron left by a disc used on carbon steel makes the surface rust. Keep the heat down too: grinding stainless steel without discoloration covers how.
• Aluminum and other non-ferrous metal. Garage Welding finds 60 or 80 grit "often easier to control than very coarse grit," on a non-ferrous-rated, load-resistant disc with light pressure, so the picker starts aluminum there even for heavy work. Whitby's grain is top-coated ceramic or zirconia, which resists loading on soft, gummy metal.
• Cast iron. Garage Welding uses 40 to 60 grit for cleanup and shaping, on a disc labeled for ferrous metal, and warns about the dust.

Speed: the disc must be rated for the grinder

OSHA's rule for abrasive wheels, 29 CFR 1910.215(d)(1), is that before a wheel is mounted "the spindle speed of the machine shall be checked ... to be certain that it does not exceed the maximum operating speed marked on the wheel." Type your grinder's rated speed from its nameplate and the speed on the disc's label, and the picker checks them. Whitby gives typical maximum speeds for Type 27 discs, which fall as the disc gets bigger:

Disc	Typical maximum speed
4 in (100 mm)	About 15,000 rpm
4 1/2 in (115 mm)	About 13,200 to 13,300 rpm; a Type 29 about 12,500
5 in (125 mm)	About 12,000 rpm
7 in (180 mm)	About 8,500 rpm

Those are typical, not a rating for the disc in your hand. Whitby's own warning is the example the table makes obvious: a 7 in disc rated for 8,500 rpm on an 11,000 rpm 4 1/2 in grinder can burst. Keep the guard on. For right-angle grinders, 29 CFR 1910.243(c)(3) limits the guard's exposure to 180 degrees and puts it between the operator and the wheel. Flap disc vs fiber disc and flap disc vs grinding wheel cover when a different abrasive does the job better, and a flap disc only finishes a weld: if it is porous or not fused, the weld defects guide is the place to start.

Sources, read on 19 October 2026: Whitby Abrasives, How to Choose a Flap Disc (grit bands, grains by metal, density, typical maximum speeds; manufacturer data); Empire Abrasives, post-weld grinding (Type 27 and 29 angles, the 40 then 60 or 80 sequence, INOX); Garage Welding, flap disc grit numbers explained; Dome Abrasives, zirconia vs ceramic for stainless; OSHA 29 CFR 1910.215 and 1910.243. The makers' figures are their own claims, and the disc's label and the grinder's manual come first.