Choosing the Right Abrasive Grit
Grit selection is the difference between efficient finishing and wasted time. Choose too coarse and you create more work for later steps. Choose too fine and you'll spend forever removing marks that a coarser grit would handle in seconds. Here's how to get it right.
Understanding Grit Numbers
Grit numbers indicate the size of abrasive particles. Higher numbers = finer particles = smoother finish but slower material removal.
| Grit Range | Classification | Particle Size | Primary Use |
|---|---|---|---|
| 36-60 | Coarse | ~250-500 µm | Heavy stock removal, shaping |
| 80-120 | Medium-Coarse | ~100-200 µm | Removing tool marks, initial finishing |
| 150-180 | Medium | ~75-100 µm | General finishing, blending |
| 220-320 | Fine | ~40-65 µm | Pre-polish prep, light scratch removal |
| 400-600 | Extra Fine | ~15-35 µm | Final finishing before polish |
| 800+ | Ultra Fine | <15 µm | Mirror finish prep, micro-finishing |
The Golden Rule: Start as Fine as You Can
The common mistake is starting too coarse "just to be safe." But coarse grits create deep scratches that require multiple subsequent steps to remove. Every step you can skip saves time.
Assess your starting surface:
- Can you feel scratches with your fingernail? → Start coarse (60-80)
- Visible marks but barely tactile? → Start medium (120-150)
- Light haze or fine scratches only? → Start fine (220+)
Grit Progression: The 50% Rule
A general guideline: each step should be roughly 50% finer than the previous. This ensures each grit can remove the scratches left by the one before.
Example progressions:
- Heavy work: 60 → 120 → 220 → 400
- Medium work: 120 → 180 → 280 → 400
- Light work: 220 → 320 → 400
Why Skipping Grits Costs Time
Jumping from 80 grit to 320 grit might seem faster, but the 320 can't remove the deep 80-grit scratches efficiently. You'll end up spending 3x longer at 320 than if you'd used 150 in between. The grit sequence exists for a reason.
Material Considerations
Different metals respond differently to abrasives. Adjust your starting grit accordingly:
Soft Metals (Aluminum, Copper, Brass, Gold, Silver)
- Scratch easily — coarse grits leave deep marks
- Can often start finer than you think
- Watch for loading (metal clogging the abrasive)
- Silicon carbide abrasives work well
Medium Hardness (Mild Steel, Stainless)
- Standard grit progressions work well
- Stainless work-hardens — don't dwell
- May need more time at each grit
Hard Metals (Tool Steel, Hardened Steel)
- Slower material removal at all grits
- May need to start coarser than expected
- Fresh, sharp abrasives essential
- Multiple light passes beat one heavy pass
Finish Requirements
Your target finish determines how far you need to go:
| Target Finish | Final Grit | Typical Ra |
|---|---|---|
| Functional (paint prep, non-visible) | 120-180 | 50-100 µin |
| Satin/Brushed | 180-320 | 20-50 µin |
| Smooth (visible surfaces) | 320-400 | 10-20 µin |
| Pre-polish | 400-600 | 5-10 µin |
| Mirror | 800+ then compound | <5 µin |
Working Direction
Change direction between grits. This serves two purposes:
- Visual confirmation — New scratches run at a different angle, so you can see when the old ones are gone
- Better surface quality — Cross-hatching creates a more uniform texture than unidirectional scratches
A common pattern: work 0° → 45° → 90° → 135° as you progress through grits.
When to Replace Abrasives
Worn abrasives don't cut — they burnish and generate heat. Signs it's time to replace:
- Cutting action noticeably slower
- Generating more heat than usual
- Glazed or shiny appearance on the abrasive surface
- Tool has worn significantly smaller
Fresh abrasives are faster and produce better finishes. Don't be penny-wise and pound-foolish.
Rubber-Bonded Grit Grades
Rubber-bonded abrasives like Cratex use letter grades rather than numbers:
| Grade | Equivalent Grit | Use |
|---|---|---|
| Coarse (C) | ~60-80 | Deburring, heavy scratch removal |
| Medium (M) | ~120-180 | General finishing, machining mark removal |
| Fine (F) | ~220-320 | Pre-polish, light finishing |
| Extra Fine (XF) | ~400+ | Final finishing, polish prep |
The rubber bond provides advantages over rigid abrasives: cooler cutting, better conformability, and self-dressing action that exposes fresh abrasive as the tool wears.
Starter Recommendation
For most finishing work, you need three grits: coarse for initial work, medium for general finishing, and fine for polish prep. Cratex Mini Kits include all three, making them an economical way to build your grit range.
Summary
- Assess first — Start as fine as your surface condition allows
- Follow the sequence — Each grit removes the previous grit's scratches
- Don't skip grits — It costs more time than it saves
- Change direction — Confirms complete scratch removal
- Match grit to finish requirement — Don't over-finish non-critical surfaces
- Use fresh abrasives — Worn tools waste time and compromise quality
Grit selection is a skill that improves with experience. Pay attention to what works, and you'll develop intuition for the right starting point and progression for any job.