Fix Bondo Pinholes: Glazing, Guide Coat & Feather Steps

Nothing sinks an otherwise clean panel like pinholes peeking through primer. Tiny voids in body filler (Bondo) show up as dark dots after sanding, then as fisheyes or craters once you spray. The fix isn’t to drown them in heavy primer—it’s to find every hole with a guide coat, glaze only what’s needed, and feather the repair into surrounding paint with a tight grit ladder. This workflow keeps the plane true, edges crisp, and your topcoat free from telegraphed defects.

Why sanding discipline matters

Pinholes are shallow but numerous. Coarse paper digs troughs that take forever to hide; soft pads “chase” each pit and dish the area. Ultra-fine grits just polish the tops while voids remain. A hard-backed, straight-stroke sequence lets you shear the surface flat, spot-glaze with the least material, and refine to a uniform matte that buries quickly under sealer and primer—without thinning edges or leaving swirl.

Tools & supplies

• Hard sanding blocks: long flat block for panels, small firm block for body lines, and a tiny block for spot work
• Silicon carbide sheets: 150, 180, 320 (400 optional for thin primers)
• Guide coat (powder or light mist), raking light, pencil for witness marks
• Glazing putty compatible with your filler/primer system; spreaders
• Degreaser/panel wipe, clean microfiber towels, masking tape
• Vacuum with brush tip, squeegee, and compressed air (dry)
• PPE: respirator, eye protection, gloves

Recommended grit sequence

• Re-level the repair area: 150 grit on a hard block to cut back to a flat, honest surface before glazing.
• Glaze-sand & shape: 180 grit to level fresh glazing putty without trenching.
• Feather for primer: 320 grit to blend the repair into surrounding paint and set an even, shallow scratch that primers bury fast.

Step-by-step

1. Map the truth with a guide coat. Degrease the repair and 3–4 inches beyond. Dust on a faint guide coat. Under a raking light, the color hangs in lows (including pinholes) and disappears from highs. Lightly pencil a witness grid so you stop the moment each zone is addressed.
2. Re-level at 150 on a hard backer. Wrap 150 grit around a rigid block and make long, straight, overlapping strokes. You’re not hunting each dot—you’re bringing the plane true so glazing stays thin. Swap sheets early; dull paper skates and polishes. Keep a dependable supply like 150 Grit Sandpaper (25-pack) so you can change early instead of pressing harder.
3. Blow, squeegee, and inspect. Vacuum, blow out dust from pores, and pull a squeegee across the surface. Dark specks that persist in the guide coat are pinholes; tiny craters you can feel with a glove snag need glaze.
4. Glaze only what needs it. Mix glazing putty as directed and squeegee a razor-thin film across the pitted zone, pressing across the scratch direction to force material into voids. Skim at a low angle to leave almost nothing on the highs. Let it cure fully.
5. Level glaze at 180—no dishes. With a clean, hard block, sand 180 in straight strokes that bridge glaze and surrounding filler. Change stroke angle slightly from the 150 step so leftover lines jump out. For batch work or larger panels, staying stocked with 180 Grit Sandpaper (50-pack) keeps the cut consistent without resorting to pressure.
6. Re-apply guide coat and re-check. A quick dusting tells you instantly if any low specks remain. If you still see isolated pits, repeat a micro-glaze and a short 180 pass. Don’t spot-dig with fingertips—that’s how saucers form.
7. Feather to surrounding paint at 320. Expand your sanding zone 2–3 more inches and blend the repair into the old finish with 320 on a hard block. One or two even passes should erase 180 lines and leave a fine, uniform matte. For show-panel consistency, finish this step with 320 Grit Sandpaper (100-pack) so the last pass cuts identical to the first.
8. Edge protocol. Keep tape on sharp body lines until the final 320 blend; then remove the top layer of tape and make one feather-light pass parallel to the line to unify sheen without thinning the edge.
9. Clean and seal if required. Vacuum, tack, and panel-wipe per your system. If you’ve exposed mixed textures (bare metal, filler, and old paint), use the manufacturer’s recommended sealer before primer for even absorption and color.
10. Prime smart, then final-sand. Apply two or three controlled coats of primer with proper flash. After cure, block the primer at 400–600 (not the raw repair) to prep for base/clear.

Special cases

Endless pinholes after sanding: The filler may have been over-mixed or whipped air in. Let it cure fully, re-level at 150, and glaze in two thin passes rather than one heavy smear.
Cold shop, slow cure: Putty sands gummy and smears into scratches. Warm the panel to spec, extend cure, and use fresh paper.
Thin OEM clear near the repair: Keep 320 off the edge until the last feather pass. It’s better to leave a hair more texture than to strike through a body line.
Deep voids disguised as pinholes: Probe with a razor. If they’re deeper than a skim can bridge, spot-fill with filler, re-block at 150, then resume glaze steps.

Pro tips

• Hard backing wins. Foam pads and fingertips carve dishes that show under light and take forever to hide.
• One direction per grit. Straight at 150, gentle diagonal at 180, straight again at 320 makes leftover scratches obvious—and removable.
• Change sheets early. Loaded paper skates and polishes, creating gloss islands that fight primer.
• Guide coat = truth. Re-dust between grits and stop the moment witness color disappears evenly—extra strokes only thin film.
• Feather wider than you think. Blends that end abruptly telegraph under metallics; expand your 320 zone for invisible transitions.
• Record your stack. Grits, stroke directions, putty brand, cure times, temp/humidity—repeatability beats guesswork.

Aftercare

• Let primer and subsequent coats reach full cure before aggressive washing or sun bake.
• If tiny specks reappear after first primer pass, stop and spot-glaze those micro-pits; don’t try to bury them in heavy primer.
• Before base/clear, scuff the primer per your system (often 600) and wipe meticulously to prevent fish-eyes.

FAQs

• Why did pinholes show up? Air entrained during mixing, over-worked filler, solvent contamination, or sanding dust trapped in voids.
• Can high-build primer fill pinholes? It hides some, but they often “print” back. Glaze is faster, cleaner, and uses less film.
• Can I DA this whole process? A DA with a firm pad can help on flats, but finish each grit by hand on a hard block to keep planes true and blends invisible.
• Is 220 a good middle grit? It can be; if your glaze is stubborn, a brief 220 pass between 180 and 320 works. Keep strokes straight and re-guide-coat.
• Do I need sealer? If you expose mixed textures or cut into base, yes—use the system’s sealer before primer for uniform color and holdout.

Video

Closing

Pinholes are inevitable—visible repairs aren’t. Work with a guide coat, keep the backing hard, and climb a disciplined ladder: 150 → 180 → 320. Glaze thin, change sheets early, feather wider than you think, and seal when you expose mixed textures. Follow this plan and your primer will lay flat, your base will cover evenly, and your clear will reflect a repair that simply disappears.