Flap discs grind like a wheel and finish like a sanding disc, which is why they have largely replaced both for weld dressing on steel and stainless. The abrasive cloth flaps wear back as they work, exposing fresh grain, so the disc keeps cutting instead of glazing.
Type 27 is flat and used for grinding on flat surfaces. Type 29 is conical and angled, which lets it get into fillets, corners and inside radii where a flat disc cannot reach. The conical shape also gives better control and a larger contact area at an angle.
Grain type determines performance and cost. Zirconia alumina is the standard for steel and stainless — good cut rate, reasonable life, moderate price. Ceramic grain cuts faster and cooler and lasts longer, which makes it cheaper per unit of work on hard and stainless steels despite the higher price. Aluminium oxide is for general light work.
Grit follows the same logic as sanding: 36-40 for heavy stock removal, 60-80 for general weld blending, 120 and finer for finishing. Backing type — fibreglass or plastic — affects durability and heat resistance. Maximum RPM must match the grinder.
