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Spot and fix sanding defects

Almost every sanding defect has one of three causes: a skipped grit, too much pressure or a disc that no longer cuts. Find what you are seeing – below it you will find where it comes from and what helps now.

The film

Pigtails in the paint? Disc clogging?

Almost always one of three causes: a skipped grit, too much pressure or a disc that no longer cuts.

5 defects

On the sanded surface

Pigtails, scratches, waves, sanded-through edges – visible under raking light or, at the latest, with guide coat.

Defect 01

Pigtails and arcs in the surface

Also: Swirl marks · Pigtails · Pigtails in the paint · Arcs from the random orbital sander

How to spot it

Small, even arcs, usually only visible under raking light or after painting.

Where it comes from

  • A grit was skipped – the fine grit can no longer remove the deep scratches left by the coarse one.
  • Too much pressure: the random orbit is braked, and the pad just spins instead of oscillating.
  • Dust under the disc because there is no dust extraction or the disc is clogged.

What helps now

  • Go back to the grit that caused the pigtails and work up in steps – never more than doubling the grit per step.
  • Apply guide coat before every change: only move to the next grit once it has gone everywhere.
  • Connect dust extraction, change the disc, let the weight of the machine do the work.

Grits to put it right

Read the full sequence:Car & paint Body filler

What helps

In depth in the guides:How to avoid pigtails and sanding scratches Using a random orbital sander: pressure, speed, pace

Defect 02

Scratches from the coarse grit are still there

Also: Scratches in the paint · Scratches after sanding · Visible sanding marks

How to spot it

Individual straight scratches, clearly deeper than the rest of the scratch pattern – guide coat stays in them.

Where it comes from

  • The jump to the next grit was too big, for example from P120 straight to P320.
  • A lump of paint or filler on the disc scratches deeper than any grain.
  • The surface was not wiped between grits, and a coarse grain was still lying on it.

What helps now

  • Start with the grit that reliably removes the scratch and work cleanly through the sequence from there.
  • Check the disc: lumps and shiny patches mean change it, do not carry on.
  • Wipe between steps and check with guide coat.

Grits to put it right

Read the full sequence:Car & paint Body filler Primer

What helps

In depth in the guides:How to avoid pigtails and sanding scratches Can you sand out scratches in car paint? What grit sandpaper for car paint?

Defect 03

Sanded through – primer, wood or chipboard is exposed

Also: Sand-through · sanded through · Sanded through veneer · Bare edge

How to spot it

The colour suddenly changes on edges and curves; on veneer, a different wood grain or glue shows through.

Where it comes from

  • The machine was tilted, or put down or lifted off with the pad running – then the edge of the pad cuts in at one spot.
  • Started too coarse where only a scuff sand was needed.
  • Sanded over the edge, where the disc flips up.

What helps now

  • Stop immediately: sanded-through veneer cannot be recovered; on paint it means priming and building up again.
  • Do edges, corners and curves by hand with a block or non-woven abrasive, not with the machine.
  • Put a soft interface pad under the disc so the edge of the backing pad does not cut in – and set the machine down flat before switching it on.

Grits to put it right

Read the full sequence:Wood Car & paint

What helps

In depth in the guides:Sanding veneer without sanding through Sanding edges, profiles and curves Using a random orbital sander: pressure, speed, pace

Defect 04

The filler repair shows through

Also: Filler is visible · Transition shows through · Filler edge in the paint

How to spot it

An edge you can feel with the flat of your hand before you see it – after painting it shows as a shadow.

Where it comes from

  • Sanded only on the filler instead of well beyond it – the edge remains as a step.
  • A soft pad or bare hand followed the bump instead of levelling it.
  • No primer surfacer applied: the paint sinks into the fine scratches and pores.

What helps now

  • Sand flat with P80 on a hard, long block or a hard backing pad – always beyond the repair.
  • Feather the transition out flat with P120 and P180 – the wider, the less visible.
  • Remove the scratches of the previous steps with P240, then primer surfacer – it seals what the paint would otherwise trace.

Grits to put it right

Read the full sequence:Body filler Car & paint

What helps

In depth in the guides:Sanding a filler repair invisibly: feathering the edges What grit for sanding body filler?

Defect 05

The surface goes wavy, edges get rounded

Also: Dips from sanding · Uneven surface · Rounded edges

How to spot it

Under raking light or with a long straightedge, shallow hollows show up; edges that should be crisp have gone soft.

Where it comes from

  • The backing pad was tilted or put down with the machine running.
  • Stayed in one spot for too long instead of working in overlapping passes.
  • A soft pad on a surface that needs to be flat – it follows the unevenness instead of removing it.

What helps now

  • Work flat: hard or medium backing pad, long hand sanding block for surfaces that need to be level.
  • Move slowly in overlapping passes, each pass covering half of the previous one.
  • Make hollows visible with guide coat – where it stays, you need filler, not more sanding.

Grits to put it right

Read the full sequence:Body filler Car & paint Wood

What helps

In depth in the guides:Using a random orbital sander: pressure, speed, pace Hand sanding: with a block, in strokes, criss-cross Sanding a filler repair invisibly: feathering the edges

4 defects

On the disc and while sanding

The disc clogs, falls off or does not lie flat – or the material smears instead of sanding.

Defect 06

The disc no longer cuts, it smears

Also: Sandpaper clogged · Disc loaded up · Disc no longer cuts

How to spot it

Same pressure, same speed – and still nothing happens; the surface gets warm, and there are shiny patches or lumps on the disc.

Where it comes from

  • Worked without dust extraction: the dust clogs the grains before they are blunt.
  • Paint or filler was not fully cured – soft material gums up any disc.
  • Too much pressure: the disc gets hot and smears.

What helps now

  • Clogged is not worn out: blow it out or clean it with an abrasive cleaning stick on the running machine.
  • If the grains are rounded or broken off and the surface is shiny, only a new disc will help.
  • Connect dust extraction; abrasive mesh extracts across the whole surface and clogs more slowly.

Read the full sequence:Car & paint Wood Body filler

What helps

In depth in the guides:When is a sanding disc worn out? Mesh sanding discs vs. regular sandpaper Sanding dust: extraction and masks

Defect 07

The disc falls off the backing pad

Also: Hook-and-loop won’t hold · Sanding disc comes off · Hook-and-loop pad worn out

How to spot it

Even a new disc will not stay on; the hooks on the backing pad look flattened or discoloured.

Where it comes from

  • The hook-and-loop on the backing pad is worn – the hooks break or melt when the disc shifts.
  • Dust and resin are stuck in the hook-and-loop.
  • The machine was put down or lifted off while running freely, or guided over edges.

What helps now

  • Clean the hook-and-loop with a soft brush or compressed air – that is often enough.
  • If it is flattened or discoloured: fit a replacement backing pad with the same diameter and hole pattern.
  • Put an interface pad in between: it wears instead of the backing pad and is quicker to replace.

What helps

In depth in the guides:Sanding disc no longer sticks to the pad Which backing pad for which abrasive?

Defect 08

Plastic smears and fuzzes instead of sanding

Also: Bumper smearing · Plastic going soft · Fuzz instead of dust

How to spot it

Instead of sanding dust you get fuzz and shiny smeared patches, and the disc clogs within seconds.

Where it comes from

  • Plastic conducts heat poorly: high speed and heavy pressure soften the surface.
  • The mould release agent was not wiped off and got rubbed in by the abrasive.
  • Started too coarse – anything coarser than P180 is rarely needed on plastic.

What helps now

  • Small orbit, low speed, light pressure – better to change the disc once more often.
  • Wipe new parts with plastic cleaner before the first sanding.
  • Smooth scratches with P180, refine with P240 and P320, matt down with P400 before painting; edges and swage lines with non-woven abrasive.

Grits to put it right

Read the full sequence:Plastic

What helps

In depth in the guides:Sanding plastic: preparing bumpers properly What grit before painting?

Defect 09

The discs curl or do not lie flat

Also: Sandpaper curling · Disc not lying flat · Brittle paper

How to spot it

The disc does not lie flat on the backing pad, sands unevenly and comes off more easily.

Where it comes from

  • Damp storage: paper absorbs moisture from the air and curls.
  • Heat – next to a radiator, in a van in summer – ages the bond and the hook-and-loop.
  • Opened packs were left open or not stored flat.

What helps now

  • Store dry and at room temperature, lying flat in the original packaging, rolls standing upright.
  • Put slightly curled sheets flat under a weight for a day.
  • Throw out badly curled or brittle discs – they sand unevenly and are more likely to fall off the backing pad.

What helps

In depth in the guides:How to store sandpaper properly When is a sanding disc worn out?

4 defects

Only after paint or finish

Rough wood, paint that will not stick, blisters, rust that comes back – the fault was in the substrate.

Defect 10

Wood feels rough after the first coat

Also: Raised grain · Wood rough after varnish · Surface rough after oiling

How to spot it

The surface was sanded smooth and is rough again after the first coat.

Where it comes from

  • Water-based varnish or stain raises the wood grain.
  • Oil raises the grain too – after the first application.
  • Sanding dust was still in the pores and got sealed in.

What helps now

  • Raise the grain before the final sand: dampen the surface, let it dry, go over it with the final grit – then the finish will not raise the grain again.
  • Lightly de-nib between coats with P320 or fine non-woven abrasive – just scuff, do not sand through.
  • Vacuum the dust thoroughly, including from the pores, and wipe with a tack cloth.

Grits to put it right

Read the full sequence:Wood

What helps

In depth in the guides:Sanding wood before oil or varnish: which final grit? Pine, spruce, larch: sanding softwood

Defect 11

The new paint won’t stick or beads off

Also: Paint peeling · Paint beading · Craters in the paint · Poor adhesion

How to spot it

The fresh coat pulls together, beads off in places or can later be lifted with a fingernail.

Where it comes from

  • Shiny spots were left – anything still shiny has not been keyed and gives the paint nothing to hold on to.
  • Polish, wax, silicone or release agent was spread into the scratches while sanding instead of being removed first.
  • The surface was not degreased after sanding or was touched with bare hands.

What helps now

  • Matt down evenly until no shiny spot remains: flat areas with P400 to P600, edges and swage lines with non-woven abrasive.
  • Clean first, then sand – wipe down old furniture and new plastic parts beforehand.
  • Vacuum the dust, degrease with silicone remover, wipe with a tack cloth and do not touch the surface afterwards.

Grits to put it right

Read the full sequence:Primer Plastic Wood

What helps

In depth in the guides:What grit before painting? Keying and repainting painted furniture Sanding plastic: preparing bumpers properly

Defect 12

The rust comes back in the same spot

Also: Rust bubble returns · Rust bleeding through · Rust under the paint

How to spot it

Weeks or months after the repair, the paint bubbles again in exactly the same spot.

Where it comes from

  • Rust was left in the pores – under a bubble, the rust is almost always bigger than the bubble.
  • Painted over the rust instead of removing it down to bare metal.
  • The bare metal was left exposed too long: flash rust forms within hours.

What helps now

  • Sand with P80 until there is bright, silver metal all round.
  • Feather the edge into the old paint with P120 and P180, and key the surrounding area with P240 to P320.
  • Degrease and prime the same day; if the metal has rusted through, no amount of sanding will help.

Grits to put it right

Read the full sequence:Metal Car & paint

What helps

In depth in the guides:Sanding and repairing rust bubbles on a car Sanding metal: rust, welds, paint prep

Defect 13

Blisters in the paint over a filler repair

Also: Paint bubbles · Blisters over filler · Filler blistering

How to spot it

After painting, small blisters appear over a filled area – and nowhere else.

Where it comes from

  • The polyester filler was wet sanded: it is porous, soaks up the water and releases it later.
  • The primer surfacer was not fully cured when painted.
  • Fresh filler was sanded too early – soft material gums up and traps moisture.

What helps now

  • Always dry sand filler, with dust extraction – wet sanding is for the finished paint, not the substrate.
  • Primer surfacer may be wet sanded, but must then dry out completely.
  • Open up the area again, dry sand down to a sound base and build it up again.

Grits to put it right

Read the full sequence:Body filler Car & paint

What helps

In depth in the guides:Wet or dry sanding? Sanding primer surfacer: wet or dry? What grit for sanding body filler?

Not what you are seeing?

Tell us what you are working on and what happened – a photo helps. We will tell you which grit and which abrasive will save the spot.

Ask us directly →Abrasive finder