The right sandpaper grit after grinding welds on auto body steel is 80 grit. Use it to remove grinder marks and bring the weld area to clean, bare metal. From there, shape any body filler with 80 and then 120 to 180, sand primer with 220 to 320, and finish with 400 to 600 grit before paint, following the product data sheets.
Quick Answer
Grind the weld flush, sand with 80 grit to remove the grinder scratches, then step up through finer grits. Do not jump from a coarse grinding disc straight to a fine paper, and do not start finish sanding until the weld is flush, clean, and free of pinholes. Grit numbers below are common starting points, not a rule. Your filler, primer, and paint instructions come first.
Key Takeaways
- Use 80 grit right after grinding to clean up scratches and prepare the metal.
- Shape body filler with 80 grit, then refine with 120 to 180 and finish at 180 or finer.
- Sand primer with 220 to 320 grit, then 400 to 600 before basecoat if the paint system calls for it.
- Change one step at a time. Big jumps in grit leave scratches that show through paint.
- Prime bare metal promptly so the sanded weld does not rust.
Sandpaper Grit After Grinding Welds: Quick Sequence
This table shows a common order of work after a weld has been ground down. It follows the grit ranges published by 3M for Bondo body filler and by an abrasives supplier, Red Label Abrasives. See the sources at the end of this guide.
| Stage | Typical grit | What it does |
|---|---|---|
| Grind the weld flush | Grinding disc or flap disc, about 40 to 80 grit | Removes the weld bead height quickly |
| Clean up grinder marks | 80 grit | Leaves clean bare metal with a scratch pattern filler and primer can grip |
| Shape body filler (if used) | 80, then 120 to 180 | Levels the repair and blends it into the panel |
| Finish sand before primer | 180 and finer | Removes coarse scratches from the filler stage |
| Sand primer | 220 to 320 | Levels primer and gives the topcoat something to grip |
| Final sanding before basecoat | 400 to 600, depending on the paint system | Removes primer scratches before color |
Note: Primer and paint makers publish their own sanding grits. If a data sheet gives a different grit, follow the data sheet.
Step 1: Grind the Weld Flush Without Thinning the Panel
Sandpaper is not the tool for knocking down a weld bead. Use a grinder with a grinding disc or a coarse flap disc to take the bead down close to the panel surface. Stop just before you reach the panel. Finishing the last bit with 80 grit is safer than cutting into the metal.
- Keep the grinder moving and use light pressure. Heat warps thin body panels.
- Let the panel cool between passes if it gets hot to the touch.
- Check progress with your fingertips or a straightedge. A ridge you can feel will usually show in the paint.
- Do not grind so much that the panel becomes thin around the weld.
For more on this stage, see how to grind welds flush on a car body and the flap disc grit chart.
Removing Old Welds or Damaged Metal First
If the repair starts with cutting out rusted or damaged metal, plasma cutting is one way to do it, and grinding is another. Either way, the cut edges and old weld remains need cleaning up before you weld in the new metal. The guide on plasma cutting vs grinding to remove old welds compares the two methods.
Step 2: Use 80 Grit to Remove Grinder Marks
After grinding, go over the weld and the area around it with 80 grit on a sanding block or a dual-action sander. 3M’s directions for Bondo Original Filler tell you to sand the damaged area with 80 grit to strip it to bare metal, so 80 is also a sensible match for this stage.
- Sand until the weld area and the surrounding metal look uniform, with no deep gouges.
- Feather the edge of the surrounding paint so you do not leave a hard step.
- Wipe the area clean with wax and grease remover before you apply anything.
- If the weld is smooth and flush with only tiny low spots, you may be able to go straight to primer instead of filler.
Step 3: Shape Body Filler Only If You Need It
Filler is for small low spots, not for hiding a rough weld. Apply thin layers over clean bare metal and follow the filler maker’s directions for mixing and cure time. Once cured, 3M lists 80 grit for rough shaping and 180 and finer for finishing. A common middle step is 120 to 180 on a block to level the repair and blend the edges.
Make sure the weld underneath is sound first. Pinholes or porosity in the weld will show up as pinholes in the filler. See how to use body filler over welded auto body panels for the application steps.
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Step 4: Prime and Sand the Primer
Bare metal rusts quickly, so prime it as soon as the sanding and cleaning are done. Use a primer that suits bare steel and your paint system. Once the primer has cured as its data sheet says, level it with 220 to 320 grit. Red Label Abrasives gives 220 to 320 as the usual range for this stage.
A light guide coat of contrasting color helps here. Sand until the guide coat is gone. Any color left behind marks a low spot that needs more attention.
Step 5: Final Sanding Before Paint
The last pass before basecoat is usually 400 to 600 grit, according to Red Label Abrasives. The exact grit depends on the primer and paint system, so check the paint maker’s data sheet. An example progression from the same source is 80, then 120, then 220, then 400. Whatever you choose, change grit in modest steps so each paper removes the scratches left by the last one.
Grit Mistakes to Avoid
- Finishing with a coarse grit. Deep scratches from a grinding disc or very coarse paper can show through filler, primer, and paint.
- Skipping grits. A large jump leaves scratches the finer paper cannot remove.
- Sanding in one direction only. Work the area in crossing directions and check with a guide coat.
- Sanding through the primer. Bare metal showing through needs to be re-primed.
- Skipping the cleaning step. Grinding dust, oil, and fingerprints cause adhesion problems.
- Mixing metals. Do not use abrasives that were used on steel on aluminum, and the other way around.
Troubleshooting Problems After Sanding Welds
| Problem | Likely cause | What to do |
|---|---|---|
| Scratches show through the paint | A coarse grit was left in place or grits were skipped | Go back to the grit that made the scratches and work up through the sequence |
| Weld outline visible under primer | Bead not ground flush, or the filler is uneven | Reshape with a block and recheck with a straightedge or guide coat |
| Pinholes in filler over the weld | Porosity or contamination in the weld | Grind the area out, repair the weld, and clean before refilling |
| Rust forms around the weld | Bare metal was left unprimed or moisture was trapped | Remove rust to bright metal and prime promptly |
Pinholes often trace back to weld quality. See what causes weld contamination in auto body work and what a good weld bead profile looks like.
Safety and Dust Control
- Wear eye or face protection, hearing protection, and gloves when grinding and sanding.
- Use a respirator rated for dust when sanding filler, primer, and ground metal, and ventilate the work area.
- Keep grinder sparks away from fuel lines, upholstery, and other flammable material.
- Check the safety data sheets for your filler and primer and follow the maker’s precautions.
- Coated or galvanized metal can release harmful dust and fumes when ground or heated. Use extra ventilation and protection.
Reference Photos: Plasma Cutting Setup for Panel Removal
These photos cover plasma cutting setups that some shops use to remove damaged metal before repair welding. They are included as shop reference for the cutting stage, not for sanding.
Frequently Asked Questions
What grit sandpaper do I use after grinding a weld on a car?
Start with 80 grit to remove the grinder marks and clean up the metal. Then work up through finer grits depending on whether you use filler: 120 to 180 on filler, 220 to 320 on primer, and 400 to 600 before basecoat. Follow the product data sheets for your materials.
Can I put body filler directly over a ground weld?
Yes, if the weld is sound, flush, and sanded to clean bare metal, and the surface is clean and dry. Follow the filler maker’s directions. Do not use filler to hide a rough bead or a weld with pinholes.
What grit should I finish with before primer?
3M’s Bondo directions call for finishing filler with 180 grit and finer before primer. Your primer’s data sheet may recommend a specific grit, so check it before you prime.
Can I use 36 grit on a weld?
36 grit is a coarse grit that suits heavy stock removal, but it leaves deep scratches. If you use it, follow with 80 grit and work up through finer papers so the scratches do not show through the paint.
Should I sand wet or dry after grinding welds?
Dry sanding with a block or DA sander is common for metal prep, filler, and primer. Wet sanding is more common on finer grits before paint. Follow your primer and paint data sheets, and avoid water on bare metal that is not primed right away.
Related reading: abrasive grit size for metalworking, how to grind welds with an angle grinder, and what affects repair weld strength.
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Conclusion
After grinding welds on auto body steel, 80 grit is the starting point for clean-up, followed by a steady climb through finer grits. Shape filler with 80 and 120 to 180, sand primer with 220 to 320, and finish with 400 to 600 before basecoat, adjusting to your product data sheets. Keep the weld sound, the metal clean, and the grit changes small, and the repair will blend in under paint.
Sources
- 3M Bondo Original Filler: directions for sanding to bare metal with 80 grit, rough shaping with 80, and finishing with 180 and finer.
- Red Label Abrasives: Sandpaper for Auto Body Repair: grit ranges for metal prep, filler shaping, primer sanding, and pre-paint sanding.
