How do I make a sanding block, and does it really matter?

Sanding by hand is not about the abrasive. It is about pressure distribution. Fingers apply pressure at four small points, so they cut faster at those points and produce a wavy surface that looks flat until light falls across it or a gloss finish goes on. A block spreads the pressure across its whole face and keeps the surface true.

What you'll need

• Flat offcut of timber or MDF, roughly palm-sized
• Cork sheet, felt or dense foam
• Contact adhesive or PVA
• Craft knife and a straightedge
• Offcuts of the mouldings you need to sand (optional)

Step by step

1. Cut a block that fits your hand.Around 110 by 70mm, and thick enough to grip comfortably — 25mm or so. Size it so a standard sheet of abrasive divides into it neatly with enough to fold up the sides, which for most sheets means about a third of a sheet per wrap.
2. Make sure the face is flat.Check with a straightedge. A block with a twisted or bowed face reproduces its own shape in the workpiece, which defeats the whole purpose. Plane or sand the face flat before you use it.
3. Glue a soft facing to it.A few millimetres of cork, dense felt or firm closed-cell foam. This is what makes the difference: it grips the abrasive so it does not slip, and it lets the abrasive follow small variations without letting the block rock.
4. Chamfer the edges slightly.A sharp corner on the block digs a line into the work when the block tips. Break the arrises with a plane or a file.
5. Make a hard block as well.For levelling filler, flatting a finish and truing an edge you want a block with no give at all — bare timber, or timber with the paper stuck down. Soft and hard blocks do different jobs and both are worth having.
6. Make a profile block for mouldings.Take an offcut of the actual moulding — skirting, architrave, glazing bead — and use it as the block, with abrasive wrapped over it. It matches the profile exactly, so the shape stays crisp instead of being rounded away by fingers.
7. Make a curved block for curved work.For an inside curve, a dowel or a length of pipe of the right radius. For an outside curve, a shaped offcut, or a block faced with thick soft foam that conforms as you press. For a small hollow, a strip of abrasive used like a shoeshine cloth.
8. Wrap the paper so it cannot slip.Fold the paper round the block and grip the ends against the sides with your fingers, or cut a slot in each end of the block and tuck the paper into it. A block with a spring clamp at each end is the traditional design and works well.
9. Use it correctly.Long strokes, along the grain, with the block flat. Let the weight of your hand supply most of the pressure. Lift at the end of a stroke rather than dragging back under pressure, which rounds the ends.

Tips

• Keep the block clean. Grit picked up on the face transfers straight to the workpiece and puts a scratch across everything.
• Make several blocks in different sizes. A small one for tight work and a long one for keeping a long edge straight are both far better than one compromise size.
• For sanding a flat panel dead flat, a longer block bridges the low spots and cuts only the high ones — exactly the same principle as a long plane.
• Number or mark your blocks by the grit you keep on them, so you never accidentally put coarse grit through a finished surface.

Alternatives

• Bought rubber sanding block: Cheap, ergonomic and holds a third-sheet with a clamp at each end. Perfectly good for general work and better than a shop-made one for comfort over long sessions.
• Cork block: The traditional plasterer's and decorator's block. Slightly compliant, grips paper well, and inexpensive. A good default.
• Sanding sponge: Foam with abrasive bonded to all faces, in various grits. Excellent for mouldings, curves and getting into corners, and it flexes into shapes a block cannot. Not for producing a flat surface.
• Hard block with paper glued on: Spray adhesive holding the paper to a flat piece of MDF gives an unyielding surface that will level filler and flatten a panel better than anything compliant.

Questions people ask