Why Your Scratches Won't Clear: Check Contamination Before Technique

You advanced to the next grit, ground for the usual time, and the scratches from the previous step are still there. The instinct is to grind longer or press harder. Most of the time that's the wrong move, and sometimes it actively makes things worse.

The grinding ladder assumes each paper cuts at its nominal particle size. In a working lab that assumption fails constantly. The gap between what the label says and what the abrasive surface actually is causes the majority of "scratches won't clear" complaints. So the rule is simple: run the contamination differential before the technique differential. Time and load are valid levers only after you've confirmed the paper, the platen, and the sample are clean.

Here's the checklist, roughly in order of how often each one is the culprit.

1. Loaded or glazed paper

On soft alloys — aluminum, copper, lead, tin, solder — swarf welds into the abrasive surface. The paper looks fine but has stopped cutting. The tell: the surface flattens and burnishes, but the prior step's scratches never clear, no matter how long you grind.

Fix: replace the paper, reduce load, and for chronically soft alloys consider alumina paper or diamond film, which load less than silicon carbide.

2. Coarse particles on your finer papers

Shared grinding stations are the worst case here. A 400-grit paper that picked up loose coarse particles from a 180-grit slurry session is no longer 400 grit in any meaningful sense. It cuts at mixed grit and leaves deep random scratches that the operator reads as "the previous step wasn't finished." Adding time makes it worse — you're cutting new deep scratches with every pass.

Fix: fresh paper at every transition on shared or visibly-used stations, and rinse the platen, not just the sample. Dried slurry on the platen re-wets and migrates onto the new paper.

3. Stray oversize grit in fresh paper

Abrasive particle-size distributions are not perfectly tight, especially on lower-cost paper. A single oversize particle puts one deep scratch into an otherwise clean grind. The pattern is distinctive: one or two deep unidirectional scratches against a surface that otherwise looks right.

Fix: discard that sheet and run a fresh piece. Don't burn ten minutes adjusting parameters to fight one rogue particle.

4. A dirty platen under clean paper

Fresh paper on a contaminated platen still drags coarser debris into contact, either through the paper or around its edges. Rinse and wipe the platen before mounting every new sheet. It takes ten seconds.

5. Cross-step carryover

Sample edges, gloves, brushes, rinse bottles, a sleeve resting on the next station's cloth — coarse grit travels on all of them. The symptom is scratches at random orientations that don't match any single step's grinding direction.

Fix is workflow discipline, not parameters: rinse the sample at every transition, change gloves when they're gritty, and dedicate brushes per grit range.

6. You may be a full grade off without knowing it

FEPA P-grades and ANSI grit numbers diverge badly at the fine end. P1200 is roughly ANSI 600 — not ANSI 1200. A US-trained operator working from European-stocked paper (or the reverse) can be a full step off the intended ladder and the sequence will feel mysteriously too coarse or too fine at one position.

Fix: check which standard is printed on the paper and convert before you blame your technique.

7. Water quality

Hard water leaves mineral deposits that act as contamination on the next finer step. The symptom is subtle: a faint, widespread haze that survives even a clean polish. A deionized water rinse on the final two steps usually clears it.

The diagnostic order

Symptom pattern	First suspect
Surface burnishes, scratches never clear	Loaded/glazed paper
Deep random scratches, worse with more time	Coarse contamination on finer paper
One or two deep scratches, surface otherwise clean	Stray oversize particle
Random-orientation scratches, no step matches	Cross-step carryover
Ladder feels one step wrong	FEPA vs ANSI mix-up
Faint haze after final steps	Water quality

Only after fresh paper, a rinsed platen, a rinsed sample, and a confirmed grit standard should you reach for more time or more load — and at that point, "premature transition" (genuinely not enough time at the step) is a fair diagnosis, because it's the only suspect left.

Most labs that adopt this order find that the fix was a two-dollar sheet of paper, not a new procedure.