Fix Poly Drips & Sags: Level, Feather, Reflow Cleanly

We’ve all seen it: a beautiful coat of polyurethane ruined by a few drips on an edge or a sag that dried like a frozen wave. Don’t strip the whole piece—this is a surgical repair. With the right sanding sequence and a careful reflow coat, you can level the defect, feather the transition, and lay down a fresh, flawless film that blends invisibly.

Why Sanding Matters

Drips and sags are localized thickness problems. If you try to hide them with more finish, they often telegraph worse. Sanding resets the topography: you knock down the hump, unify the surrounding sheen, and set a consistent scratch field so the next coat wets evenly. The trick is using hard backers for flatness, light pressure to protect the surrounding film, and balanced grit jumps so you remove ridges without digging trenches.

Tools & Materials

• Sharp razor blade or cabinet scraper (for initial nib removal)
• Hard sanding block (phenolic/MDF) and a thin foam pad (for slight contours)
• Random-orbit sander (optional) with dust extraction for larger flats
• 9×11 in silicon carbide wet/dry sheets: 320, 600, 1000
• Raking/side light and a soft pencil for guide-coats
• Vacuum with brush head, microfiber cloths, tack cloth (non-oily)
• Solvent compatible with your poly (for wipe-down) and your polyurethane topcoat
• PPE: respirator for dust/solvent, eye protection, gloves

Recommended Grit Sequence

• 320 grit — Primary leveling of drips/sags; sets flatness without deep gouges.
• 600 grit — Refines 320’s trenches; tight, uniform matte ready for reflow.
• 1000 grit — Optional pre-recoat refine on high-gloss or thin films.

Step-by-Step: Level, Feather, and Reflow

1. Let the coat fully cure. Poly that gums or rolls isn’t ready. Wait until it powders under light sanding. Use raking light to map drips/sags and pencil a faint guide-coat around each defect.
2. Shave the peak. Hold a razor blade slightly skewed and draw it across the drip to shave the crown. Keep it flat to avoid digging shoulders. Two or three whisper passes should knock the highest ridge down.
3. Block-sand at 320. Wrap 320 around a hard block and make short, overlapping strokes across the defect, then with the grain to blend. For dependable cut at this stage, use 320 grit (25-pack). Sand until the drip/sag is flush and your pencil map is gone uniformly.
4. Feather the transition with 600. Re-mark a small pencil halo 1–2 inches beyond the repair. Step to 600 grit (50-pack) on a block for flats or a thin foam pad on gentle contours. Your aim is a uniform matte that erases 320 trails and softens the edge of the repair so the next coat disappears.
5. Optional clarity pass at 1000. On dark colors or high-gloss builds, a brief pass with 1000 grit (100-pack) further tightens the scratch field, reducing the chance of witness lines under a thin recoat. Stop as soon as 600’s marks vanish in raking light.
6. Dust removal & solvent wipe. Vacuum, then wipe with a clean, lint-free cloth and a finish-compatible solvent (per your poly system). The surface should sheet uniformly in a light water-break test—no beading or greasy islands.
7. Reflow coat. Lay a thin, even coat of polyurethane over the entire panel (or at least a natural break, like a stile/rail). Maintain a wet edge and avoid flooding the repaired zone. The fresh film should self-level across your feathered area without telegraphing an edge.
8. De-nib & final coat (as needed). After cure, inspect under raking light. If dust nibs are present, kiss the surface with 600–1000, clean, and apply a final thin coat.

Special Cases

Edge drips on verticals: Tape a 1–2 inch guard below the drip, shave the bead, then block 320 with very light pressure, feathering up the face. Finish 600 ? (1000) before a controlled recoat. Work in thinner layers to prevent repeats.
Waterborne poly: Tends to show witness lines if you jump too coarse. Stay disciplined: 320 ? 600 (? 1000). Watch for grain raise; plan a light de-nib between coats.
Oil-modified poly: Longer open time helps self-level, but drips can be tougher. Don’t overwork—sand flat, feather, and reflow in a thin pass.
Curved moldings: Use a thin foam pad at 600/1000 and short, with-profile strokes to avoid flats. Finish by hand with the grain.

Pro Tips

• Hard backer = flat repairs. Fingers make scallops. Use a firm block for 320; reserve foam only for slight curves.
• Pencil + raking light is your cheapest QC. Advance grits only when the previous map is 100% gone.
• Thin recoats win. Heavy passes flood the repair and sag again. Two thin coats beat one thick one.
• Temperature & humidity control sag risk. Work within the product’s window and avoid drafts that skin the surface too fast.
• Retire dull paper early. Dull sheets polish, hide deeper scratches, and can glaze the film.

Aftercare

• Allow full cure before handling or setting items on the surface—soft films imprint easily.
• Clean with mild soap and water; avoid strong solvents on young finishes.
• For future touch-ups, label the final prep grit on masking tape (e.g., “Repair stop: 600/1000”).
• If a small sag appears within the first hour on fresh coats, you can sometimes tip-off gently with a quality brush to level it before it sets.

FAQs

• Can I start at 220 instead of 320? 220 cuts fast but risks trenching the film, making a visible flat. 320 is the safer start for cured poly.
• Do I need to wet-sand? Dry 320 is ideal for leveling. Wet-sanding at 600–1000 reduces clogging and keeps things cool, especially on large panels.
• Will one thin recoat cover the repair? Usually yes if the feather is clean. If you can still see a witness line, de-nib lightly at 600–1000 and apply one more thin coat.
• What if I sand through to wood? Stop, feather the edge, spot-seal (e.g., with dewaxed shellac if compatible), then rebuild the poly film with light coats.
• Why do drips keep happening? Too much finish, cool temps, or heavy edges. Strain the finish, use a quality brush/foam applicator, work thin, and tip-off edges last.

Watch the Repair Flow

Closing: Don’t panic-strip. Shave the peak, block at 320, feather at 600 (? 1000 for gloss), clean thoroughly, and reflow a thin, even coat. With firm backers, disciplined grits, and good light, poly drips and sags disappear like they were never there.