Because the point carries almost all of the work. The apex is where the operator drives the tool into corners and against edges, so contact pressure concentrates on a very small area while the two rear corners and the middle of the sheet barely touch anything.

The result is a sheet that looks nearly new but has stopped cutting where it matters. Operators often respond by pressing harder, which glazes the worn tip and heats the workpiece without removing material.

Hook and loop backing exists largely to answer this. Lifting the sheet, turning it and re-seating it brings an unworn corner to the front, and a single triangular sheet therefore delivers three useful working points before it is spent.

Not reliably, and this catches buyers out. Although every delta sheet is broadly triangular, manufacturers differ in the exact size and proportions of the triangle, and more importantly in the pattern of dust extraction holes punched through it.

A sheet that is close but not correct will still stick to the platen, yet its holes will not line up with the extraction ports. Dust then stays on the surface, clogs the abrasive and scratches the work.

Match sheets to the platen dimensions and hole pattern of the specific tool. Some ranges include multi-hole or universal patterns intended to serve several tools, and those are the practical choice for a workshop running mixed brands.

Aluminium oxide is the general-purpose grain for bare timber and painted surfaces. It fractures to expose fresh edges as it works and is tough enough for the intermittent, cornering contact that detail sanding involves.

Silicon carbide is sharper and more brittle, cutting cleanly through varnish, lacquer and filler, and it is the usual grain where the finish itself is being levelled rather than the wood beneath.

Ceramic grain suits harder work and heavier removal, holding its edge on metal and dense hardwoods. It costs more per sheet but survives the concentrated tip loading better, which changes the arithmetic on anything other than light work.

They matter a great deal. Sanding dust that stays between the abrasive and the workpiece does two things: it fills the space between grains so the sheet stops cutting, and it drags across the surface leaving scratches finer grits were meant to remove.

Extraction through aligned holes lifts that dust away as it is generated. The sheet keeps cutting at its rated grit, lasts noticeably longer, and the surface stays clean enough to judge as you work.

On sealed surfaces such as paint and varnish the effect is stronger still, because those materials produce soft dust that packs into the abrasive quickly and glazes it, sometimes within a minute of continuous work.

Cleaning is worth trying first when the sheet is loaded rather than worn. A rubber abrasive cleaning stick held against the running sheet pulls packed resin, paint and dust out of the grain and can restore much of the cutting rate.

That works for loading, not for wear. If the grain itself has rounded over, no cleaning will bring the edges back and the sheet is finished.

Distinguishing the two is simple enough: a loaded sheet looks shiny and discoloured with material sitting in the surface, while a worn sheet looks pale and smooth with the grit visibly flattened. Given the tip wear pattern, rotating the sheet is usually the faster answer to both.

Skipping grits is the usual reason. Each grade should remove the scratch pattern the previous one left, and jumping several grades leaves deep scratches that the finer abrasive smooths at the surface but does not eliminate. They reappear as soon as stain or finish goes on.

Tilting the tool is a close second. Working on the edge of the platen concentrates pressure on a line and cuts a visible ridge, particularly on softwood.

Trapped debris completes the list. A single coarse grain dislodged from an earlier sheet, or a chip of hardened filler caught under the platen, will score the surface repeatedly until it is found and removed.

No, and using one that way is expensive. The triangular platen exists to reach places larger tools cannot, and its small area means a low removal rate over any open surface.

Working a delta sander across a broad panel consumes sheets rapidly, takes far longer than an orbital or belt sander would, and produces an uneven result because the operator has to overlap many small passes.

The efficient sequence is to strip open areas with a larger sander and bring the detail sander in for corners, rebates, glazing bars, spindles and the junctions where the bigger machine cannot reach. Sheets then last as intended and the finish is consistent.