Sandpaper: P-Grade Grits, Between-Coats Technique, and What Actually to Buy in the UK

A homeowner finishes fitting MDF skirting and architrave on a Saturday afternoon. Filler in the nail holes, ten minutes with a folded sheet of P40 off the bargain bin at the petrol station, straight to undercoat. Two coats later, every nail hole is a halo of swirl marks that won't disappear under topcoat, every length of skirting catches the light like sandblasted glass, and the architraves look fuzzy where the grain raised under water-based primer. The fix is to strip back, refill, sand properly with a graded sequence, prime, undercoat, and topcoat. A weekend's lost. The right thing was a single own-brand assorted-grit pack at £4 – £7 and ten minutes' reading on grits.

Sandpaper is the cheapest consumable on a paint job and the one that decides whether the finish looks professional or homemade. Pick the right grit. Don't skip steps in the progression. And know which type of paper goes on which surface, because using the wrong abrasive on bare wood will raise the grain and make things worse.

What sandpaper actually is

Modern sandpaper isn't sand. It's a backing sheet (paper, cloth, film, or open mesh) coated with grains of a hard mineral bonded with adhesive. The four common minerals each have a job:

• Aluminium oxide is the workhorse. Tough, hard, and the default grain on every general-purpose sheet. Use it on bare wood, primed wood, filler, and most metals.
• Silicon carbide is sharper but more brittle than aluminium oxide. It cuts paint film cleanly without loading up, which is why it's the standard grain on "wet-and-dry" papers and on specialist between-coats products like Oakey Between Coats. Use it on cured paint, lacquer, and metal polishing.
• Garnet is a softer natural grain that breaks down as it cuts, exposing fresh edges. The result is a slightly cleaner cut on bare timber, particularly on hardwoods being prepared for stain. Less common in UK retail than the other three.
• Ceramic is the toughest grain, used in heavy-duty stripping and floor sanding belts. Overkill for hand-prep on extension woodwork.

The other distinction the bag won't tell you is open coat vs closed coat. Closed-coat paper has grains covering 90-95% of the backing surface. It cuts faster but loads up (clogs) on resinous or soft material. Open-coat paper has grains spaced across about 50-70% of the surface, leaving voids for debris to escape. It cuts slightly slower on hard timber but is the right choice for sanding wood filler, soft pine, and any half-cured paint film. If a sheet says "stearate-coated" or "non-loading", it's open-coat with a soap-like additive that further resists clogging. Buy this for between-coats work.

The grit numbering system: FEPA P-grade vs CAMI

UK sandpaper is sold under the FEPA P-grade system (the European standard, identifiable by the "P" prefix on every sheet: P80, P120, P240). The US market uses the CAMI system, which uses the same numbers without the prefix. Up to about P220 the two systems are practically interchangeable. Above P240 they diverge sharply, and a US guide telling you to "use 600 grit between coats" means something very different from a UK guide saying "use P600".

Practical implication: read the prefix. If you're buying paper from a UK retailer for a UK job, work in P-grades and use any of the published UK grit charts. Ignore American YouTube videos that say "320 grit" without specifying the system. P-grades are the tighter, more consistent system anyway. The narrower particle-size distribution under FEPA gives a more uniform scratch pattern than CAMI at the same nominal number, which is what you want for finish work.

The grit progression rule: never jump more than 50%

The single biggest beginner mistake is skipping grits. Going from P80 straight to P220 leaves the deep scratches from P80 visible under the paint film, because P220 only removes the surface gloss without touching the trenches the coarse grain cut. Two coats of satinwood later, you can still see the P80 marks raking across the skirting in low light.

The fix is the 50% rule: never increase the grit by more than 50% in a single step. After P80, the next jump is to P120 or P150. After P120, to P180. After P150, to P220. The exception is when going to a finer grit purely for de-nibbing between coats, where you don't need to remove deep scratches because you're sanding a paint film, not bare timber.

Standard progressions for the common jobs:

The "rough sawn vs planed" distinction matters on bare timber. Planed-all-round (PAR) softwood from a merchant is already smooth enough to start at P150 or P180. Sawn timber off a forklift load needs P120 first. MDF from the same merchant needs almost nothing because the factory finish is fine to begin with. Treating all timber the same wastes paper and time.

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Aluminium oxide, silicon carbide, garnet: matching the paper to the job

Most own-brand "general purpose" sheets at Wickes, Screwfix, and Toolstation are aluminium oxide. That covers 80% of paint-prep work on an extension. The other 20% needs something different.

Aluminium oxide is the right choice for: sanding bare timber, sanding wood filler, sanding primer and undercoat, prepping MDF, and any general material removal. It's tough enough to keep cutting through resin pockets and durable enough to last several pieces of skirting per sheet at the right grit.

Silicon carbide is the right choice for: sanding cured topcoat between layers, wet-and-dry de-nibbing of gloss or satinwood (used dry on the paint film), polishing metal, and any work where you need a sharp grain that doesn't load up. The Oakey Between Coats range is the trade-standard product here. £17 – £19 for a 10m roll is more than the price of three packs of own-brand, but the stearate coating means it won't clog mid-pass, and one roll covers a full extension's worth of skirting and architrave between-coats work.

The fourth case is the wet-and-dry trap. Wet-and-dry papers are silicon carbide on a waterproof backing, designed to be used with water as a lubricant on cured paint or metal. Used as designed, on a sealed or painted surface, they give a near-perfect finish. Used wrong, on bare unsealed timber with water, they raise the wood grain into a fuzzy mess that's worse than what you started with.

Sheets, rolls, discs, sponges, mesh: the format question

Sandpaper comes in five mainstream formats, each for a different working pattern.

Sheets (typically 230 x 280mm full sheet or 115 x 280mm half) are the default for hand sanding on a block. Tear or fold to fit, wrap around a cork or rubber sanding block, and work the surface. £4 – £7 for a mixed-grit 10-pack from any major UK merchant. Sheets give the most paper per pound but require more setup than discs.

Rolls (5m or 10m, single grit) are sheets in continuous form. Tear off what you need, wrap the block, work the surface, throw away. Best for jobs where you'll burn through one specific grit (a long run of skirting at P180, for example) and want to standardise on a single product. £3 – £4 for a 5m own-brand roll, £5 – £10 for a 10m. Premium specialist rolls like Oakey Between Coats sit at £17 – £19.

Discs are circular sheets sized to match a specific power sander: 125mm for the most common random-orbital sanders, 150mm for larger ones, plus assorted shapes for detail and mouse sanders. Hook-and-loop (Velcro) backing is universal on modern discs. £6 – £8 for a Bosch or Trend 10-pack. Disc work is faster than hand sanding for flat areas and is the only economic format for a long extension run, but the disc machine doesn't reach into corners or onto profiles.

Sponges are foam blocks coated with abrasive grains. They flex around curves and sit on profiled mouldings (ogee skirting, chamfered architraves, scotia) where a flat block bridges the high points and misses the hollows. £5 – £7 for a multipack from Bosch or Toolstation. Reusable until the foam tears or the grain wears off. The right tool for moulding work.

Mesh abrasives are the premium format. Mirka Abranet is the leading UK product: a net of glass-fibre threads coated with aluminium oxide, with thousands of small holes for dust extraction. Used on a sander with a vacuum hose, mesh produces near dust-free sanding because the dust passes straight through the disc into the extraction. £21 – £26 for a 10-pack of 125mm discs at trade prices. Roughly twice the lifespan of a paper disc, so the per-shot cost works out broadly comparable to mid-range paper discs in heavy use, with the dust benefit on top. The trade-off: mesh edges fray on sharp profiles, and the velcro pad on the sander runs hotter than with paper discs (the Mirka recommendation is to fit a foam interface pad between the velcro and the mesh disc to absorb heat).

For an extension build, the format mix is typically:

• One pack of mixed-grit sheets for general filler-and-prep work.
• One 10m roll of P180 or P220 for the long skirting runs.
• One pack of P240 or P320 sponges for between-coats on architrave profiles.
• One pack of 125mm random-orbital discs at P120 if you're machine-sanding doors or larger panels.

That's £25 – £35 in consumables and covers the woodwork on a 30m² extension end-to-end.

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Sanding blocks: why bare-handed sanding ruins flat surfaces

Sanding a flat panel with a folded sheet held in your hand seems like it should work. It doesn't. Your fingertips concentrate pressure into points, and the points dig localised depressions into soft material while the spaces between your fingers leave higher zones untouched. Run your hand across a "hand-sanded" door panel and you'll feel the dips. They show up under topcoat as wavy reflections.

The fix is a sanding block: a rigid reference surface that distributes your pressure evenly across the abrasive's full footprint. The block sits on the high points of the surface and cuts them down before touching the low points, which is exactly what you want.

Block types matter:

• Cork blocks (£2 – £4 from any merchant) are the trade standard. Slight give, won't dent the work, holds a folded sheet securely. Use for dry sanding bare timber, primer, and topcoat.
• Rubber blocks with hardness around Shore A 60-70 are firmer than cork. Best for keeping a clean edge on flat panels and for the discipline of feather-sanding filler patches. Slightly more expensive at £4 – £6.
• Foam blocks are softer than cork. Use only where the surface is shaped and you want the block to follow the curve. Not the right choice for flat work.

Bare hand sanding is fine for one job: working a profiled moulding where no block fits the contour. For everything flat, use a block.

Between-coats sanding: P240, P320, or skip entirely?

This is the most-debated topic on UK painting forums and the place a homeowner is most likely to get conflicting advice. Here's what's actually going on.

Between-coats sanding has two purposes: removing dust nibs and brush hairs that landed in the wet film, and creating mechanical adhesion ("key") for the next coat. The first goal is universal. The second goal depends on the paint chemistry.

Modern water-based satinwood and gloss are designed to bond chemically between coats applied within their recoat window (typically 4-6 hours for water-based). You don't need a mechanical key to get adhesion. Sanding between coats is purely cosmetic, removing the imperfections in the previous film so the next coat goes on flat. P320 is the right grit for this. You're abrading the surface gloss, not removing material. Light pressure, one pass, dust off, recoat.

Modern oil-based satinwood and gloss also bond chemically within their recoat window (16-24 hours), but the surface cures hard enough that the next coat genuinely benefits from a key. P240 is the working choice. Slightly coarser than the water-based ideal, but the key matters more here.

Long-cured paint film (a previously painted surface that's been on the wall for months or years) needs both a mechanical key and a clean surface. P180 is the right grit, used aggressively to break the surface gloss, then P240 to refine. This is "scuff-sanding" in trade terminology. It's a different operation from de-nibbing between fresh coats.

Skipping between-coats sanding entirely is acceptable on water-based paints applied with a roller in good conditions where the surface comes out flat and free of nibs. It's not acceptable on brushed work, where brush hairs and small particles land in the film as it skins.

The other call is what to dust with before recoating. Tack cloths (sticky resin-impregnated cheesecloth) are the traditional answer and work well on oil-based finishes. They are the wrong tool for water-based paints because the resin in the tack cloth can transfer to the surface and cause "fisheyes" (small circular dewetting marks where the next coat refuses to flow over the contaminated patch). Use a microfibre cloth, dry, for water-based work. Two-step (microfibre then tack) is fine for oil-based topcoats.

What to buy

The format question above answers most of the buying decision. Here's the specific shopping list for an extension build, in priority order.

Default pick: own-brand mixed-grit 10-pack at £4 – £7. Wickes, Screwfix Essentials, B&Q Prep, or Toolstation Triton-branded packs all sit in this price range. P80 to P240 covered in a single pack. Good enough for general prep, filler-feathering, and pre-primer sanding. Buy two packs at the start and you're covered for the basic work.

Heavy work: own-brand 10m roll at £5 – £10. Pick the grit you'll use most (typically P180 or P220 for skirting prep). Tear off as needed. The 10m option is roughly half the per-metre cost of a 5m roll. Stick to 5m if you want to spread your money across multiple grits.

Between-coats specialist: Oakey Between Coats roll at £17 – £19. Silicon carbide, stearate-coated, doesn't clog. Available P120, P180, P320 (buy P320 for water-based topcoats, P180 if you're cutting back a thick old film before recoating). Trade decorators run this through every job. The own-brand alternative for between-coats is the silicon carbide wet-and-dry sheets at any major retailer, used dry, but Oakey is faster and cleaner.

Profile work: foam sanding sponges at £5 – £7. A multipack covers most useful grits. Reach into ogee mouldings on skirting, chamfered architrave edges, and scotia profiles where a block can't sit flat. The grit on a sponge wears slower than on a paper sheet because the foam absorbs the working pressure rather than transmitting it directly to the abrasive.

Machine discs: Bosch or Trend 125mm 10-pack at £6 – £8. Buy hook-and-loop discs to match your random-orbital sander. Get a P120 pack for general material removal and a P220 pack for finishing. Branded discs cut more consistently than no-name imports and last longer.

Premium pick: Mirka Abranet 125mm at £21 – £26. Worth the price jump only if you're machine-sanding for a full day with extraction connected, where the dust benefit and disc lifespan justify the cost. For a one-off extension build with intermittent power-sanding, the budget Bosch discs are the better economic choice.

Storage, shelf life, and the humid-shed problem

Paper-backed sandpaper is hygroscopic. The backing absorbs moisture from the air and curls or weakens once it's saturated. A sheet stored in a damp shed for a winter will tear more easily than a fresh one and may delaminate at the grain layer (the abrasive lifts off the paper as you sand).

The practical rules:

• Store opened packs indoors, ideally in a closed plastic box or zipper bag at 35-50% relative humidity.
• Keep paper-backed discs flat, not on edge. Velcro backing on edge-stored discs picks up debris that stops them seating properly on the sander.
• A 10-pack of own-brand sheets has effectively no shelf life as long as it's kept dry. Sheets bought five years ago in a sealed envelope work the same as fresh.
• Open mesh discs (Abranet, similar) are less affected by humidity because the mesh isn't paper, but the velcro backing still degrades if it gets wet.

HSE wood dust limits and why FFP3 isn't optional

Wood dust isn't just an irritant. It's a Workplace Exposure Limit (WEL) substance under HSE regulations, and prolonged exposure causes occupational asthma and nasal cancer. The numbers, set by the HSE:

Sanding a 30m² extension's woodwork generates dust well above these limits if you don't extract it. The mitigation hierarchy:

1. On-tool dust extraction. Connect a Class M industrial vacuum (NOT a household Henry without dust class certification) to the sander's extraction port. This captures most of the dust at source. Mesh discs paired with extraction approach dust-free operation.
2. FFP3 disposable respirator. £3 – £4 per mask. The minimum protection level for wood-dust sanding under HSE guidance. FFP2 is acceptable for short, intermittent sanding sessions; FFP3 is the standard for sustained work. Replace every 4-5 hours of heavy use.
3. Ventilation. Open windows and doors. A floor fan blowing outwards near the work area keeps dust moving out rather than settling.
4. Wet methods where applicable. Sanding cured paint with wet-and-dry papers using water as the lubricant produces effectively no airborne dust. Not applicable to bare timber.

What the HSE specifically forbids: dry sweeping or compressed-air clearing of wood dust. Both put settled dust back into the air at concentrations many times above the WEL. Use a vacuum, ideally an industrial one rated for fine particulates.

The plaster-dust case is different and worse. Plasterboard joints contain calcium sulphate and a small percentage of crystalline silica, which is a Group 1 carcinogen. Sanding skim plaster or plasterboard joints requires the same FFP3 protection plus ideally on-tool extraction with H or M class units.

Where you'll need this

• Decoration: every paint-grade timber surface in the extension needs sanding before primer, between primer and undercoat, and between undercoat and topcoat. Skirting, architrave, doors, door linings, window boards, and any boxed-in pipework.
• Second-fix carpentry: filling and sanding nail holes in fitted skirting and architrave before the decorator arrives. This is often the carpenter's responsibility but homeowners on self-managed jobs end up doing it themselves.
• Plastering and skim work: light sanding of skim coats before mist coat to remove trowel ridges and small surface defects. P120 only; aggressive sanding goes through the skim.
• Flooring: drum sanding solid floorboards uses the same grit progression rule but on industrial belts. The principles are identical even when the format isn't.

These same consumables turn up on any extension, loft conversion, garage conversion, or interior renovation involving paint-grade woodwork. Buy the right mix at the start of the project rather than running out mid-sand and ending up with whatever the local petrol station stocks.

Common mistakes

Skipping grits in the progression. Going from P80 directly to P220 or higher leaves the deep scratches from the coarse grit visible under topcoat. The 50% rule isn't optional. P80 to P120 to P180 to P220, in that sequence, every time on bare softwood.

Using too coarse a grit between coats. P120 between coats removes paint, raises the underlying timber, and leaves visible scratches in the next coat. P240 minimum between coats; P320 for the last sanding before the final topcoat.

Wet-sanding bare wood. Water raises the grain. The surface is rougher after wet sanding than before. Wet-and-dry papers belong on cured paint, primer, and metal. Used dry, they're fine on bare wood. Used with water, they ruin it.

Sanding flat surfaces freehand. Your fingers create high-spot pressure points that dig into the timber. Use a cork or rubber block on every flat area. Reserve freehand sanding for profiles where no block fits.

Tack cloth before water-based topcoats. The resin in tack cloths transfers to the surface and causes fisheyes (circular dewetting marks) under water-based gloss and satinwood. Microfibre cloth (dry) is the right tool for water-based work. Save the tack cloth for oil-based finishes.

Reusing dull sandpaper. A worn sheet polishes the surface rather than cutting it. The friction generates heat, melts paint film locally, and creates burnishing marks instead of a clean key for the next coat. When the paper stops biting, throw it away. A new sheet is pence.

Ignoring dust during and after sanding. Wood dust on the surface contaminates the next coat. Dust in the air is an HSE WEL substance. Run a vacuum during sanding (extraction port on the sander or floor-stand vacuum nozzle below the work) and microfibre the surface before recoating. FFP3 mask through the whole job.

Buying the cheapest pack at the petrol station. P40 from a no-brand bargain bin is often poorly bonded and sheds grain into the work. The grain becomes a scratch contaminant on the next pass with a finer paper. Spend the extra couple of pounds for an own-brand pack from a proper merchant.