Sanding Bondo Pinholes: Glazing, Guide Coat & Feathering

Nothing ruins a fresh paint job like tiny craters that show up after primer—the infamous pinholes that hide in body filler and reveal themselves under gloss. The fix isn’t magic; it’s a disciplined process that combines guide coat, smart glazing, and a controlled sanding sequence that leaves edges feathered and invisible under paint. In this step-by-step, you’ll learn exactly how to find the pinholes, fill them cleanly, and sand the repair so it disappears—without digging low spots or telegraphing scratches.

Why sanding strategy matters

Pinholes are voids. If you smear filler over them without opening and cleaning the surface, they stay trapped and reappear after primer shrinks. If you attack with grits that are too coarse, you’ll carve tracks that take forever to erase; too fine and you’ll just burnish the surface, leaving the holes untouched. The sweet spot is a progression that opens the pores, levels the glaze, and feathers the repair into surrounding OEM paint with a shallow, uniform scratch that primers love. Back every sheet with something firm so you don’t round edges or dish the repair area.

Tools & supplies

• Hard sanding blocks (phenolic/Delrin) in at least two sizes; small profile blocks for body lines
• Silicon carbide wet/dry sheets: 180, 320, 400 (600 optional between primer coats)
• Glazing putty appropriate for your primer system (2K or air-dry; follow tech sheet)
• Guide coat (powder or a light contrasting mist)
• Wax & grease remover and lint-free towels
• Masking tape (low-tack) to protect edges, trim, and panel gaps
• Raking light and a clean squeegee
• Optional: DA with a firm interface pad (refining only), straightedge, and feeler gauge
• PPE: respirator, eyewear, nitrile gloves

Recommended grit sequence

• Open & shape: 180 grit to key the filler surface and knock down minor highs around the repair.
• Refine & feather: 320 grit to level glaze and blend into surrounding paint.
• Primer-ready scratch: 400 grit to set a uniform, shallow profile that buries quickly.

Step-by-step

1. Degrease and mask the zone. Clean the panel with wax & grease remover. Mask trim and tape just shy of panel breaks so you don’t accidentally roll sharp edges.
2. Map pinholes with a guide coat. Dust a light guide coat across the filler and its halo. Under a raking light you’ll see tiny dark dots (the pinholes) and any faint low spots that still need attention.
3. Open the surface at 180. With a hard block, make light, overlapping strokes to cut the filler’s surface and erase glaze sheen. Your goal is to open the pores and flatten minor highs, not re-shape the entire repair. For predictable cut that stays flat, use 180 Grit Sandpaper (25-pack) and swap sheets as soon as they slow.
4. Blow clean and re-check. Vacuum or blow off dust, then re-apply a whisper of guide coat. If you still see clusters of dots, you’ve successfully exposed the voids—don’t try to “sand them away.”
5. Glaze only what needs it. Mix/activate glazing putty per the tech sheet. Press it firmly into the pinholes with a flexible spreader, scraping excess off so you’re not creating a wide hump. Feather the outside edge thin. Let it cure fully—rushing this step invites later sink.
6. Level the glaze at 320. Back to a firm block. Sand in straight, overlapping strokes that span the glaze and the surrounding filler/paint together. Change direction slightly (e.g., diagonal) so 180 lines stand out and disappear. For batch work across multiple spots, keep momentum with 320 Grit Sandpaper (50-pack) so you always have a fresh sheet cutting cleanly.
7. Feather into paint deliberately. Remove masking that’s too close to the repair and extend your 320 blend a little into the surrounding OEM finish, keeping pressure light. A uniform, hazy ring indicates a proper feather; glossy islands mean you haven’t touched that area yet.
8. Set primer-ready scratch at 400. Make one uniform pass with 400 on a hard block to tighten the scratch across the repair and its blend zone. This shallow, consistent texture covers quickly under primer. For large panels or multiple areas, finish with 400 Grit Sandpaper (100-pack) to maintain consistency across the job.
9. Final clean and seal. Vacuum, tack, and solvent-wipe per your system. If the repair transitions from bare metal to filler to OEM, apply an appropriate sealer or epoxy primer before high-build primer to keep absorption uniform.
10. Prime, block, and re-check. Apply thin, even coats of primer, observing flash. Guide coat the primer, block with 400–600 to ensure flatness, and verify no pinholes telegraph. If any reappear, spot-glaze, 320→400, and reprime only that area.

Special cases

Deep or clustered pinholes: Often a sign of over-mixed filler (too much air) or spreading on dusty surfaces. After opening at 180, consider a thinned glazing pass to wet the pores before normal glaze. Let it cure fully.

Edge repairs and body lines: Tape a 1–2 mm safety strip along sharp creases during 180/320 so you don’t roll the edge. Remove tape for a single, light 400 pass parallel to the line to unify sheen.

Feathers that keep shrinking back: Give more cure time between glaze and primer, and avoid heavy single coats. Multiple lighter coats reduce solvent sink that can outline the repair.

Microporosity in fresh filler everywhere: Switch to a finer spreader, apply slightly firmer pressure, and don’t overwork the mix. A thin, full skim of glaze over the area, then 320→400, usually solves uniform pinhole fields.

Pro tips

• Always use hard backing on leveling grits; fingertips create divots that print under paint.
• One direction per grit: lengthwise at 180, diagonal at 320, straight at 400. It makes leftover scratches easy to spot and remove.
• Replace paper early. Loaded sheets skate and polish instead of cutting, leaving surprise scratches later.
• Don’t chase a single crater with pressure. Open the surface, glaze, and re-level the whole zone so the repair stays flat.
• Use a raking light and guide coat. Stop the moment the witness color disappears evenly; over-sanding just thins your edges.
• Keep primer coats moderate with proper flash. Trying to bury a rough scratch in one heavy pass causes sink and mapping.

Aftercare

• Allow full cure of primer and topcoat per the tech sheet before aggressive washing or polishing.
• If you find a tiny crater after color, resist compounding it away. Wet sand 1500–2000 locally, spot repair if needed, then polish.
• Record what worked: glaze brand, mix ratio, flash times, and grit stops. Your next repair goes faster with notes.

FAQs

• Why not start at 80 or 120? Too coarse risks carving troughs around the repair. 180 opens pores and shapes without deep tracks.
• Can I DA the whole process? Use a DA only for refining with a firm pad. Block-sanding controls flatness around small repairs far better.
• Is 600 better before primer? Not necessary for build primers. 400 gives reliable bite; go 600 only for sealer or before topcoat if your system requires.
• My glaze keeps pinholing too—why? Over-mixing, trapped dust, or spreading over a glossy, un-keyed surface. Open with 180, clean thoroughly, press glaze into the pores, then level 320→400.
• What if I hit bare metal at the edge? Seal with epoxy or your system’s recommended sealer before high-build primer to prevent edge mapping.

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Closing

Pinholes are small, but the fix is big on process: open the surface at 180, push glaze into the voids, level and feather at 320, then lock in a clean 400 scratch for primer. Keep the block hard, the light raking, the sheets fresh, and your coats thin with proper flash. Follow this routine and your repair will vanish under paint—no craters, no edges, no comebacks.