Hand-Sanding Window Mullion Grids with Thin Stick Abrasives

Window mullion grids add architectural beauty but are notoriously difficult to sand because of their narrow profiles and tight intersections. Power sanders can’t reach inside corners without rounding details, making hand-sanding with thin stick abrasives the best method for clean, precise results. This guide walks you through the tools, grit sequence, and pro tips for flawless mullion prep before finishing.

Why Mullion Grids Require Precision

Mullions and muntins often measure less than an inch wide, with delicate intersections where profiles meet. Even small sanding errors can flatten curves or create shiny burnish marks that telegraph under paint. Using thin, flexible abrasives ensures control while preserving the fine detail that defines traditional window joinery.

Tools & Materials

• Thin sanding sticks or coffee stirrers wrapped with abrasive
• Small foam pad for curved sections
• Vacuum and tack cloth
• Fine wet/dry sandpaper sheets: 180 Grit (25-pack), 240 Grit (50-pack), and 400 Grit (100-pack).

Recommended Grit Sequence

• 180 grit: Removes old finish or surface oxidation from small areas.
• 240 grit: Refines surface and smooths transitions around intersections.
• 400 grit: Final scuff for paint or varnish adhesion.

Step-by-Step: Sanding Mullion Grids

1. Prepare your tools. Cut narrow strips (½–¾ inch wide) of 180 Grit (25-pack) sandpaper and wrap them tightly around thin sticks or dowels. Secure ends with masking tape.
2. Work from top to bottom. Begin with vertical mullions, sanding along the grain. Use gentle strokes to avoid flattening the corners.
3. Switch to 240 Grit (50-pack). Refine the surfaces and blend where horizontal and vertical members intersect. Use light crosswise motions to remove any high spots left by the coarse grit.
4. Use a foam pad for curves. For arched or contoured grids, wrap sandpaper around a soft pad to conform to the shape without cutting edges.
5. Finish with 400 Grit (100-pack). Lightly scuff all surfaces to uniform sheen before applying primer or finish. Vacuum dust from corners thoroughly.
6. Inspect under angled light. Uneven reflections reveal missed spots—touch up as needed before painting.

Special Cases

For older windows with lead paint, use a HEPA vacuum and proper PPE. On MDF mullions, avoid over-sanding—the fine fibers can swell under paint. For exterior mullions, seal immediately after sanding to prevent moisture absorption along tiny edges.

Pro Tips

• Use new, sharp sandpaper—dull sheets polish instead of abrade.
• Wrap abrasive sticks in both directions (clockwise and counterclockwise) to reach tight corners from either side.
• Mark each mullion as you finish to track coverage—missing even a small section can cause adhesion issues later.
• Vacuum between grits to keep corners free of debris.

Aftercare

• Prime within 2 hours of sanding to seal surfaces before humidity raises fibers.
• Use a small artist’s brush to apply primer evenly inside intersections.
• Lightly resand primer coats with 400 grit before final paint for a flawless surface.
• Maintain mullions with periodic cleaning—avoid harsh chemicals that break down paint films.

FAQs

• Can I use steel wool instead of sandpaper? No—steel fibers can rust and stain paint over time.
• Should I disassemble the window? Not unless necessary. Work carefully in place using slim tools to reach corners.
• Do I need to sand between paint coats? Yes—light 400 grit scuffing between coats ensures uniform sheen and adhesion.

Watch & Learn

Precision sanding on small window details makes all the difference. With 180 Grit (25-pack), 240 Grit (50-pack), and 400 Grit (100-pack), you can bring out crisp mullion lines and perfect corners worthy of a heritage restoration.