Smooth 3D Prints: Filler Primer & 220?600 Sanding

Fresh off the printer, PLA and ABS parts show layer lines, zits, and tiny seams that catch the light and distract from your design. You don’t need exotic chemicals to make them look injection-molded—just a disciplined sanding sequence, a couple of light coats of filler primer, and smart inspection lighting. This guide walks you through a reliable, repeatable 220 ? 400 ? 600 workflow that levels layer lines, preserves fine detail, and sets you up for paint or a natural satin finish.

Why Sanding 3D Prints Matters

Layer lines are ridges. If you jump to ultra-fine grits or polish too soon, you’ll just shine the ridges instead of flattening them. Proper sanding removes high spots, closes small gaps, and creates a uniform scratch field so filler primer can do its job quickly. The keys: use firm backers (fingers make grooves), control heat (plastics smear if you push), and verify with raking light before moving up a grit.

Tools & Materials

• Rigid sanding blocks and thin foam pads (to match flats and gentle curves)
• Wet/dry sandpaper sheets: 220, 400, 600
• Filler primer (sandable), optional spot putty for pinholes
• Microfiles/sanding sticks for tight features and vents
• Raking/side light, pencil for guide-coat, microfiber cloths
• Water tray with a drop of dish soap (for wet-sanding stages)
• Masking tape to protect crisp edges and functional tolerances
• PPE: dust mask/respirator, eye protection

Recommended Grit Sequence

• 220 grit — Primary leveling of layer lines; establishes flatness.
• 400 grit — Refines 220 scratches; blends curved zones.
• 600 grit — Final pre-primer or pre-color pass for a tight, uniform satin.

Step-by-Step: From Print Lines to Smooth Satin

1. Prep & protect. Remove supports, then mask any press-fit bores, threads, or sliding surfaces. Clean the part with mild soap and water; grit from support removal scratches more than sandpaper does.
2. Primary cut at 220. Wrap paper around a rigid block for flats and use a thin foam pad on gentle curves. Make short, overlapping strokes across the layer lines, then switch direction and finish with the part’s contours. For dependable bite at this stage, start with 220 grit (25-pack). Keep pressure light to avoid heat bloom or smearing on PLA/ABS.
3. Pencil-map & inspect. Lightly pencil a crosshatch over the surface and read it under raking light. The pencil disappears uniformly only when you’ve truly leveled—don’t move up until it does.
4. Refine to 400. Re-mark a faint pencil map and step to 400 grit (50-pack). Focus on erasing 220’s trenches and unifying curves and fillets. Use sanding sticks or microfiles for vents and recesses; finish those areas with a few with-contour strokes.
5. First filler-primer pass. Blow off dust, then lay a thin, even coat of sandable filler primer from 8–10 in. away. You’re not trying to build a thick film—just fill micro-grooves and reveal low spots. Let it flash fully per the can.
6. Read the light & spot fill. Under side light, circle pinholes and persistent ridges. A dab of spot putty levels tiny divots fast; block those areas with 400 once cured so you don’t create dishes.
7. Final pre-finish at 600. Wet-sand lightly with 600 grit (100-pack) using water with a drop of dish soap. Use a rigid backer on flats and foam only to conform, never to flatten. Stop the instant the surface is uniformly satin and the primer reads smooth in raking light.
8. Second light primer (optional). If you still see faint print ghosts, apply one more light filler-primer mist and give it a gentle 600 touch after cure. Keep films thin so details stay crisp.
9. Finish choice. For color, switch to your paint system (plastic-compatible). For a natural look, you can buff the 600 surface to a soft sheen or apply a clear designed for plastics. Always test on a hidden area first.

Special Cases

PLA vs ABS: PLA softens sooner from friction; keep strokes short and pressure low. ABS tolerates a bit more pressure and can be vapor-smoothed (acetone) in advanced workflows, but sanding + filler primer is safer and more controllable for most makers.
Fine text & panel lines: Mask delicate embossing, then feather up to the edge with 400 ? 600 to avoid rounding. Remove tape and give one or two fingertip strokes along the contour only.
Transparent or translucent filaments: Filler primer will make them opaque; if you need clarity, use a longer wet ladder (600?1000?2000) and polish compounds instead of primer—on a test scrap first.
Functional tolerances: For snap fits and bearings, sand outside the engagement area and mask the interface. After finishing, chase holes with the correct reamer or a rolled 600 strip to restore fit.

Pro Tips

• Backers matter: rigid for flats; thin foam to conform curves; avoid bare fingers which create grooves.
• Short, cool strokes: plastics hate heat. If the surface warms, pause or switch to wet-sanding.
• Pencil + raking light: cheapest QC—don’t change grits until the last scratch pattern is completely gone.
• Thin coats win: multiple light filler-primer mists beat one heavy coat that hides problems until sanding bites through.
• Sanding sticks save detail: wrap narrow strips of 400/600 on sticks or dowels for louvers, vents, and recesses.

Aftercare

• Let primer and paints cure fully before assembly; soft films mar easily.
• Clean with mild soap and water; strong solvents can craze or soften prints.
• For touch-ups later, a quick 600 wet pass and a light primer fog usually restores the finish without re-running the full ladder.
• Store parts dust-free between steps; dust nibs telegraph under thin primer coats.

FAQs

• Can I start at 120 or 150? On very coarse prints, yes—but switch to 220 quickly to avoid deep trenches you’ll chase for hours.
• Dry or wet? Dry for the 220 and 400 leveling stages; light wet-sanding at 600 keeps things cool and prevents clogging.
• Why do I see shiny islands after 600? Those are low spots or over-burnished areas. Re-map with pencil, touch those zones with 400, then 600 again.
• Does filler primer hide layer lines by itself? Only shallow ones. Think of it as a helper after proper leveling—not a magic eraser.
• What about TPU or PETG? PETG sands similarly to PLA but runs hotter—go light and wet at 600. TPU is rubbery; rely more on very light scuffing and flexible abrasives rather than aggressive leveling.

Watch the Workflow

Closing: Treat 3D print finishing like any surface prep: level at 220, refine at 400, tighten at 600, and let thin filler-primer mists do targeted filling. Keep pressure light, backers firm, and your inspection light honest—you’ll get crisp parts that look professionally made, ready for paint or a clean satin sheen.