The Science of Grit Progression
Each sanding grit creates a scratch pattern with a specific depth. An 80-grit disc creates scratches approximately 0.003 inches (3 mils) deep. A 180-grit disc creates scratches approximately 0.001 inches (1 mil) deep. The next finer grit in the sequence must be aggressive enough to reach the bottom of the previous grit's scratches and remove them — while creating its own, shallower scratches that the subsequent grit (or primer) can fill.
When you skip a grit — jumping from 80 to 180, for example — the 180-grit disc can't reach the bottom of the 80-grit scratches. It sands the surface between the scratches smooth, making the panel feel flat, but the 3-mil-deep 80-grit grooves remain at the bottom of the scratch valleys. Primer fills them temporarily, but as the primer cures and shrinks over the following weeks, it sinks into those deep scratches — revealing them as visible lines through the basecoat and clear coat. This is called sinkback, and it's the most common long-term defect caused by skipped grits.
Maximum Grit Jump Rules
The general rule: never jump more than a 100% increase in grit number. From 80, the next step should be 120 or 150 — not 180 or 220. From 320, the next step should be 400 or 500 — not 800. Each step should approximately double the grit number (halving the scratch depth) for the removal process to work.
| From Grit | Acceptable Next Grit | Too Large a Jump |
|---|---|---|
| 80 | 120 or 150 | 180+ |
| 120 | 150 or 180 | 220+ |
| 180 | 220 or 320 | 400+ |
| 320 | 400 | 600+ |
| 400 | 500 or 600 | 800+ |
| 1000 | 1200 or 1500 | 2000+ |
| 1500 | 2000 | 3000+ |
Standard Grit Sequences by Application
Body Filler to Primer
80 → 120 → 180. Shape the filler at 80, refine at 120, establish the primer adhesion profile at 180. This three-step sequence removes each grit's scratches reliably and prepares the surface for primer-surfacer in approximately 15–20 minutes per repair.
Some shops skip 120, going directly from 80 to 180. On premium fillers with tight grain structure (Evercoat Quantum1), this sometimes works because the 80-grit scratches in premium filler are shallower than in economy filler. But it's a risk — and a risk that costs 2 hours of rework if sinkback appears. The 120-grit step takes 3 minutes. The rework takes 120 minutes. The math is clear.
Primer Block Sanding
320 → 400 (→ 500 optional). The 320-grit step levels the primer surface and removes nibs. The 400-grit step removes 320-grit scratches and establishes the final scratch profile for basecoat adhesion. The optional 500-grit step further refines the scratch for waterborne basecoats, high-metallic colors, and dark colors where even 400-grit scratches can show through.
Apply guide coat before each step. If guide coat remains in scratch-line patterns after sanding at the new grit, the previous grit's scratches haven't been fully removed. Continue at the current grit until the scratch-line guide coat is gone.
Wet Sanding Clear Coat
1000 or 1500 → 2000 → compound. Start at 1000 for heavy defects (runs, sags) or 1500 for moderate texture (orange peel, nibs). Step to 2000 to refine the scratch for compounding. Each step should remove the visible scratch pattern of the previous grit — confirmed by the haze becoming finer and more uniform.
How to Verify a Grit Step Is Complete
Guide Coat Method (Dry Sanding)
Apply guide coat before sanding at the new grit. Sand with the new grit until the guide coat is completely gone from the sanded area. If guide coat remains in linear patterns (scratch valleys from the previous grit), continue sanding. If guide coat remains in broad patches (low spots), don't chase with more sanding — re-prime the low areas.
Visual Haze Method (Wet Sanding)
Wipe the wet-sanded area dry and inspect the haze under a focused light. The haze should be uniform — all scratches the same depth and direction. If you see a mix of coarse and fine scratches (some deep lines among fine haze), the coarser grit's scratches haven't been removed. Continue at the current grit or step back to the previous grit to fully level.
Cross-Hatch Method
Sand at each grit in a different direction from the previous one. Sand 320 diagonally left-to-right. Sand 400 diagonally right-to-left. If any 320-grit scratches survive the 400-grit step, they'll be visible as scratches running in the wrong direction — easy to spot because they cross the 400-grit pattern instead of aligning with it.
What Skipped Grits Look Like After Paint
Immediately after paint: Nothing. The defect is invisible. The primer filled the deep scratches, the basecoat covered them, and the clear sealed everything. The repair looks perfect in the shop.
1–4 weeks after paint: The primer and basecoat shrink as residual solvents continue to evaporate. The deep scratch valleys, which were filled with more material than the surrounding surface, shrink more. The scratches re-appear as fine lines visible under direct sunlight — especially on dark and metallic colors. The customer notices. The callback happens.
The fix: Sand off the clear coat and basecoat over the affected area, re-prime with adequate build, block with the correct grit sequence this time, and repaint. Total time: 3–5 hours. Cost of the skipped grit step that caused it: 3 minutes.
When You Can Compress the Sequence
On very small repairs (chip repairs, minor scratch fixes), the sanding area is so small that each grit step takes under a minute. Compressing from four steps to three is unlikely to cause visible sinkback because the total area of deep scratches is minimal — the primer easily fills a few 80-grit scratches across a quarter-sized chip repair. On full-panel repairs, every step matters because the scratch area is large enough that any sinkback is visible.
Back to Pro Tips
