Seam Sanding Plastic Models: 600–2000 Wet & Primer Reveal

Even the best-fitting plastic model kits hide faint mold lines and joint seams. If you paint over them, they’ll flash under your glossy finish or metallics. The fix is a controlled, wet-sanding sequence that levels seams without flattening raised detail—then a quick primer reveal to confirm perfection before color. Here’s a clean, repeatable workflow that works for aircraft fuselages, armor turrets, car bodies, figures, and sci-fi shells.

Why Seam Sanding Matters

Seams and mold lines interrupt reflections and break panel continuity, especially under gloss, metallics, or NMF (natural metal finish). Sanding removes the ridge and feathers the transition so light flows smoothly. Going too coarse gouges styrene and erases panel lines; going too fine too early just polishes the flaw. A smart 600?1000?2000 wet ladder cuts scratches down in controlled steps so primer lays flat and honest.

Tools & Materials

• Sprue cutters, hobby knife with fresh blades
• Plastic cement or thin extra-thin cement; putty or CA + talc for gap fill
• Masking tape, cotton swabs, micro-applicators
• Rigid sanding blocks/sticks and thin foam pads for curves
• 9×11 in wet/dry sandpaper sheets (600, 1000, 2000)
• Water dish with a drop of dish soap (lubricant), paper towels
• Scriber/panel line guide, rivet wheel (optional)
• Primer (surfacer) and raking light
• PPE: dust mask, eye protection

Recommended Grit Sequence

• 600 grit — Level the seam and shape feather edges.
• 1000 grit — Refine 600 scratches; unify surface.
• 2000 grit — Final pre-primer refine for a smooth reveal.

Step-by-Step: From Glue Line to Invisible Seam

1. Prep and glue cleanly. Dry-fit the parts and tack with thin cement so plastic fuses across the joint (a true plastic weld). Squeeze a tiny bead at the seam so a slight ridge forms as it cures—this sands flush without low spots.
2. Protect nearby detail. Mask panel lines, rivets, and emblems with tape “guard rails.” On curves (aircraft spines, car fenders), back your paper with a flexible pad so you don’t flat-spot the contour.
3. Wet-sand at 600. Dip paper in water with a drop of soap. With light, even strokes across the seam, level only the high ridge until it blends smoothly to the surrounding plastic. For reliable control at this stage, reach for 600 grit (25-pack). Keep the surface wet and rinse the sheet often to avoid loading.
4. Fill, then re-cut if needed. If a divot appears once the ridge is gone, spot-fill with thin CA or putty, let cure, and re-level with 600 on a small block. Feather 5–10 mm beyond the defect so you don’t make a trench.
5. Refine to 1000. Switch to 1000 grit (50-pack) to erase 600’s scratches and unify sheen. Short, light, with-contour strokes prevent flats on rounded bodies. The seam should now feel invisible under a fingertip.
6. Final pre-primer pass at 2000. A brief, gentle pass with 2000 grit (100-pack) tightens the scratch field so surfacer lays glassy. Don’t chase shine—stop when the area is uniformly matte and smooth.
7. Primer reveal. Mist on a thin coat of primer and inspect under raking light. Primer honestly shows any ghost seam, pinhole, or scratch ladder. If you see defects, spot-sand (1000?2000), reprime lightly, and re-check.
8. Re-scribe and restore detail. If sanding softened panel lines, use a scriber and a straight edge/tape to restore them lightly. Spin a rivet wheel gently where needed. Re-prime those touch-ups.
9. Ready for color. Once the primer looks uniform with no telegraphed seam, proceed with your paint system. Keep coats thin to preserve crisp lines you’ve just restored.

Special Cases

Clear parts (canopies, headlight lenses): Avoid coarse grits. Start at 1000 ? 2000 wet, then polish. Mask aggressively to protect optical areas and dip in clear if your system allows.
Soft kits & vinyl: Use lighter pressure and shorter sanding sessions; heat from friction can smear the seam. Let surfaces cool between passes.
ABS/large shells (car bodies, sci-fi): ABS is tougher—600 cuts slower but stays safer. Keep pads curved to match the panel; flat blocks can facet body lines.
Putty that keeps shrinking: Switch to CA + talc as a non-shrinking filler near panel lines and re-sand 600 ? 1000 before the 2000 pass.

Pro Tips

• Use wet sanding to keep styrene cool and to prevent clogging; add a tiny drop of dish soap as a surfactant.
• Sand across the seam to level, then finish with the body contour to blend.
• Swap sheets early—dull paper polishes instead of cutting, which leaves ghost seams that reappear after primer.
• Always use a backer (block or pad). Bare fingers make grooves and flats.
• Keep primer thin and honest. Heavy primer can hide problems until masking pulls it up or gloss coats shrink.

Aftercare

• Handle primed parts with gloves to avoid skin oils that can cause fish-eyes.
• Store subassemblies dust-free between sanding and primer to prevent nibs.
• If you pause for days, do a quick 2000-grit kiss before re-priming for best adhesion.

FAQs

• Why not start at 400? It cuts fast but gouges styrene and risks flatting detail. 600 is the safest leveler for kit seams.
• Do I need polishing compounds? Only for clear parts or ultra-gloss prep. For primer reveal, stopping at 2000 is ideal.
• How do I avoid flat spots on rounded bodies? Use curved pads, keep strokes short, and rotate your approach so pressure isn’t concentrated.
• My seam reappeared after paint—why? Either shrink-back in filler or too-fine sanding early. Back up to 600 at the defect, fill if needed, and re-run the ladder.
• Can I dry-sand? Yes on tiny spots, but wet-sanding runs cooler, clogs less, and yields a cleaner scratch field on styrene.

Watch the Workflow

Closing: Invisible seams aren’t luck—they’re a system. Fuse the joint cleanly, protect nearby detail, then run a disciplined 600 ? 1000 ? 2000 wet sequence and confirm with a thin primer reveal. Keep pads backed, pressure light, and primer honest—and your models will look like a single, flawless piece.