Hand-Sanding Window Mullion Grids with Thin Stick Abrasives

Window mullion grids bring elegance and geometry to interior and exterior windows—but their narrow, intersecting bars make sanding a real challenge. Traditional sanding blocks are too large, and power sanders can’t reach tight corners. This guide explains how to use thin stick abrasives for precise hand-sanding of mullion grids, achieving smooth, even results without damaging profiles or glass.

Why Mullion Sanding Requires Precision

Each mullion or muntin separates window panes and often features narrow profiles or delicate edges. Paint buildup or rough primer can ruin symmetry if not sanded carefully. By using thin stick abrasives cut from fine sandpaper sheets, you can clean and refine these small surfaces while maintaining sharp corners and clean joints.

Tools You’ll Need

• Thin wood or plastic sanding sticks (¼-inch wide)
• Utility knife or scissors (for cutting strips)
• Masking tape to protect glass edges
• Tack cloth or microfiber cloth
• Wet/dry silicon carbide sandpaper sheets

Recommended Grit Sequence

• 220 grit: For cleaning primer or removing rough edges.
• 400 grit: For smoothing and refining contours.
• 800 grit: For polishing before final coating or glazing.

Step-by-Step: Sanding Window Mullions

1. Protect glass surfaces. Apply masking tape along both sides of each mullion to prevent accidental scratching while sanding.
2. Create sanding sticks. Cut narrow strips of 220 Grit (25-pack) sandpaper and wrap them tightly around thin wooden sticks or coffee stirrers. Secure ends with tape for a snug fit.
3. Start light sanding. Move the stick along each mullion, following its length. Maintain light, consistent pressure to remove rough primer or small paint runs.
4. Refine the surface. Switch to 400 Grit (50-pack) strips for smoother transitions. Focus on corners and intersection points to maintain even sheen.
5. Final polish. For top-quality finish or glazing prep, use 800 Grit (100-pack) sandpaper wrapped around a soft foam stick. This ensures a perfectly uniform texture for final painting.
6. Clean thoroughly. Remove all masking tape and wipe mullions with a tack cloth to eliminate dust before finishing or glazing.

Special Cases: Painted vs. Stained Mullions

For painted mullions, prioritize smoothness—paint reveals every imperfection. For stained or clear-coated wood, focus on even scratch direction to avoid visual distortion under finish. If mullions are factory-primed MDF, limit sanding to 400 grit to avoid breaking through the primer layer.

Pro Tips

• Use thin foam between the stick and paper for slightly curved mullions.
• Change sanding strips often—fine details require sharp grit.
• Always sand in long, full strokes; short motions can create uneven shine under light.
• Finish edges by hand using a folded paper corner instead of powered tools.

Aftercare

• Inspect mullions after painting and lightly scuff any drips with 800 grit once dry.
• Clean dust immediately to prevent adhesion issues for glazing compounds.
• Maintain humidity around 45–50% during finishing to reduce grain swelling.

FAQs

• Can I use nail files instead of sanding sticks? Yes, but only fine-grit nail files (400–600) for final touch-ups.
• What’s the best motion? Always sand lengthwise along the mullion, never across the grain or edge.
• Do I need to reprime? If you expose raw wood or MDF, spot-prime those areas before the final coat.

Watch & Learn

Summary: Hand-sanding window mullion grids is all about control and precision. By cutting sandpaper into narrow sticks and following a 220–800 grit progression, you can smooth every edge without damage—achieving crisp, professional results ready for paint or glazing.