Abrasives · 2 min read
Sanding walls, drywall and filler: which grit?
Joint compound with P120 and P180, ridges on smooth plaster with P80 to P120 – never sand into the paper facing. And how to keep the dust down, by machine or by hand.
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Which grit for which surface
Joint compound on drywall seams is soft: P120 takes off ridges and trowel marks, P180 leaves the surface ready to paint. Going finer achieves nothing under wall paint – the paint covers the scratch pattern.
Only sand smooth plaster where ridges or filler residue stand proud, with P80 to P120. Textured and roughcast plaster are not sanded, otherwise the texture is gone. Firmly bonded wall paint needs no sanding before repainting; take off drips with P120.
Don’t sand into the paper facing
Plasterboard has a paper facing. If you sand through the filler into it, it roughens and the fibres stand up under the paint. So sand only the filler, with light pressure and held flat. If the paper does get roughened: prime and skim thinly, don’t keep sanding. Filling thinly means less sanding – and less chance of catching the paper.
Sandpaper that doesn’t clog
Gypsum dust is very fine and quickly clogs ordinary sandpaper – it then just glides over the surface. Abrasive mesh stays open because the dust is extracted through the weave; for the random orbital sander, mesh comes as a disc (PRO NET 150). By hand, use the hand sanding block with hook-and-loop strips: the long block bridges small hollows, and a vacuum on the extraction port takes the dust away directly.
By machine or by hand
For single seams, the hand block is enough. On large areas, a random orbital sander with extraction saves a lot of work: medium speed, set it down flat, guide it without pressure. If it tips onto its edge, it sands a hollow into the soft filler.
For whole rooms and ceilings there are long-reach drywall sanders (“giraffe” sanders) with a sanding head that is usually 225 mm. KORNDEX doesn’t stock these machines or their discs; if you fill only now and then, a random orbital sander and a hand block will do.
Sanding walls without dust
Entirely dust-free isn’t possible, but low-dust is: extraction on the machine or block, plus a vacuum of dust class L or M – gypsum dust quickly clogs a household vacuum’s filter and partly passes through it. Cover the floor, tape plastic sheeting over doors, ventilate. Whatever still rises is held back by an FFP2 mask.
FAQ
How can I tell whether the wall is flat enough?
With raking light: hold a lamp flat against the wall. Ridges, dents and trowel marks then cast shadows you don’t notice in normal light – but they look exactly like that under the paint. Also feel the seams with the flat of your hand.
Do I need to prime filled areas before painting?
Yes. Filler absorbs differently from paper facing and plaster; without primer, seams and repairs show through the wall paint as matt or shiny patches. Sweep or vacuum off the sanding dust thoroughly first.
Can I wet sand joint compound?
Not with sandpaper. Small ridges on fresh filler can, however, be smoothed with a damp sponge – that creates no dust, but on larger areas it doesn’t replace sanding.
Terms in this article
All terms in the sanding glossary →The whole sequence at once
3 grits, 1 × smallest pack each: PRO NET 150 (10 pcs) P80 · P120 · P180.
incl. VAT, plus shipping
