How do I get a scratch out of an acrylic bath?
The fingernail test that tells you whether a scratch will sand out, the grit sequence that removes it without leaving a hollow, and when to stop.
- Difficulty
- intermediate
- Time
- 45 min
- Read
- 4 min
Short answer
If your fingernail catches in it, it needs wet-sanding; if it does not, polishing compound alone will take it out. Wet-sand only the scratch with 1000 then 1500 then 2000 grit, each at right angles to the last, then compound and polish back to gloss. Work small, keep everything wet, and stop as soon as the scratch has gone.
Acrylic is solid colour all the way through the sheet, which is why it can be sanded and polished back in a way that enamel cannot. There is no coating to break through. The two things that go wrong are sanding too coarse and taking a dish out of the surface, and stopping the grit sequence too early so a sanding haze replaces the scratch.
What you'll need
- Wet-and-dry abrasive paper: 1000, 1500 and 2000 grit
- A small sanding block or a cork block
- Fine polishing compound and a plastic polish
- Soft microfibre cloths
- A spray bottle of water with a drop of washing-up liquid in it
Step by step
- Do the fingernail test.Run a fingernail across the scratch. If it does not catch, the scratch is shallow and compound alone will remove it — go straight to the polishing stage. If it catches, it needs abrasive first.
- Check you are on acrylic, not enamel.Tap the bath with a knuckle away from the scratch. Acrylic sounds dull and gives slightly; steel and cast iron ring and feel cold and hard. Never sand enamel — you go through the glaze to metal and cannot get back.
- Mask around the scratch.Tape off a border a few centimetres around it. This stops the sanding creeping outward and gives you a defined patch to blend afterwards.
- Start with the finest grit that will work.For most scratches start at 1500 and only drop to 1000 if the scratch is not shrinking. Wrap the paper round a small block so the pressure is flat rather than through a fingertip — a fingertip digs a hollow.
- Keep it soaking wet throughout.Spray water on constantly. Dry sanding acrylic loads the paper, heats the plastic and puts in deeper marks than the scratch you started with.
- Sand in one direction only, then change direction with each grit.Sand the scratch out with straight strokes in one direction. Move to the next grit and sand at right angles to the last direction. When the previous grit's marks have all disappeared under the new direction, that grit is done. This is how you know when to move on rather than guessing.
- Finish the sanding at 2000 grit.The patch should look uniformly matt and slightly milky, with no visible scratch and no coarser lines running through it. If a line is still visible, go back a grit rather than trying to polish it out.
- Compound the patch back to gloss.Fine polishing compound on a folded microfibre, firm circular pressure, working outwards past the masked border into the surrounding surface so the repair blends rather than sitting as a shiny disc.
- Finish with plastic polish and wash the bath out.The final polish evens the gloss. Then wash the bath with detergent and rinse — sanding slurry and compound residue are slippery.
Common mistakes
- Starting at a coarse grit to be quick — Every grit's marks have to be removed by the next one. Starting at 400 grit turns a two-minute job into an hour of sanding.
- Sanding with the paper folded under a fingertip — The pressure concentrates and you sand a hollow. A block keeps the cut flat.
- Sanding a deep gouge that has gone through the acrylic layer — On a thin bath a deep gouge can reach the reinforcing backing. Fill that with a two-part bath repair filler instead and treat it as a chip repair.
If it doesn't work
The scratch has gone but the patch is a dull cloudy circle
Cause: The grit sequence stopped too coarse, or a grit was skipped — Fix: Go back to 2000 grit wet, work the whole patch until uniform, then compound harder and longer. Cloudiness is always an unfinished sanding sequence, never a polishing failure.
The repaired area is glossy but distorted, like a lens
Cause: Too much material removed in one spot, usually from finger pressure without a block — Fix: This cannot be polished out. Feather the surrounding area with 2000 grit over a much wider zone to spread the transition, then re-polish. Accept it will be visible at a low angle.
The surface has fine crazing around the mark
Cause: A solvent has been used on the acrylic — Fix: Nothing removes crazing — it is cracking within the plastic. Polishing improves the appearance slightly. Do not use solvent on acrylic again.
It looks perfect when wet and dull when dry
Cause: Sanding marks still present and being filled by water — Fix: Always assess a repair dry. Continue the grit sequence and polish further.