Flush Grind Weld Beads on Mild Steel (60–220 Grit Guide)

Welds should disappear into the parent metal—not leave low spots, hard ridges, or grinder swirls that show after paint. The challenge on mild steel is knocking a bead flat without carving a trench (“undercut”) or overheating the panel. This guide gives you a safe, repeatable workflow that pairs controlled grinding with a hard-backed sanding sequence so the joint reads seamless under raking light and finish.

Why sanding strategy matters

Angle grinders are fast, but they’re also imprecise: a soft flap wheel rides the bead, digs next to it, and leaves low shoulders. Go too coarse and you plow grooves you’ll chase for hours; go too fine and you just polish the crown while the bead still stands proud. The solution is to reduce the bead mechanically (with control), then switch to a hard-backed sheet sequence that keeps pressure distributed: typically 60 → 120 → 220. That ladder shears the crown, blends the field, and sets a shallow, even scratch that hides under primer—without polishing the steel slick.

Tools

• Angle grinder with flat (not cupped) flap disc or hard wheel; die grinder for tight corners
• Straightedge and raking light to check flatness
• Hard sanding blocks: long flat block for panels, small rigid block for flanges and inside corners
• Silicon carbide sheets: 60, 120, 220 (9×11 in)
• Layout fluid or marker for witness color
• Deburring tool/files for sharp edges
• Vacuum with brush tip, microfiber/tack cloths
• PPE: safety glasses/face shield, gloves, hearing protection, respirator

Grit sequence

• Shear & level: 60 grit—just coarse enough to flatten the crown and bring the weld flush with the field.
• Blend & refine: 120 grit—erase 60-grit tracks, remove discoloration, and unify the plane.
• Prime-ready tooth: 220 grit—tight, even matte that hides under primer and doesn’t polish the steel slick.

Step-by-step

1. Prep the joint and protect edges. Deburr spatter with a file. Mask adjacent finished faces and any machined lands. Color the area with layout fluid/marker—witness color tells you what you’re removing.
2. Reduce the crown with control. Use a flat flap disc and keep it flat to the work, skimming the bead with light pressure. Stay off the shoulders; don’t “edge” the wheel. Work in short passes and cool the part as needed. Stop while the bead is barely proud—sanding will finish the flattening.
3. Shear the last high with 60 on a hard block. Wrap a fresh sheet and take straight, overlapping strokes across the joint so the block bridges weld and parent metal. Witness color should vanish first on the crown; the shoulders should dull evenly. Change paper the instant the cut slows. Start with dependable stock like 60 Grit Sandpaper (25-pack) so you can swap early instead of pressing harder.
4. Read the plane; don’t chase lows. Hold a straightedge over the joint under raking light. If tiny blue/black islands (low spots) remain, make two full-width passes with the hard block at 60—never fingertip in the lows, which creates a trench you’ll see after primer.
5. Blend at 120 to erase grinder & 60 tracks. Change stroke angle slightly (gentle diagonal), then return to lengthwise strokes for the last passes. Keep pressure feather-light and the block fully supported. For larger runs and consistent cutting, stage mid-quantity sheets like 120 Grit Sandpaper (50-pack) at the bench.
6. Detail tight features safely. For inside corners or flanges, use a tiny rigid backer so you bridge the joint. If the disc couldn’t reach a fillet, file first, then re-enter at 120 on the block—files keep faces square; soft pads round them.
7. Set prime-ready tooth at 220. One or two even passes at 220 tighten the 120 pattern into a fine, uniform matte. This is your stop for primer or a conversion coat. Finish consistently with a fresh supply like 220 Grit Sandpaper (100-pack) so the last part sands like the first.
8. De-dust and inspect. Vacuum with a brush tip, then wipe. Raking light should show a single plane with no shiny trenches along the weld. If bands remain, re-enter one grit coarser on a hard block and take two full-width passes—don’t spot-dig.
9. Seal or prime within the window. Bare mild steel flashes fast. Apply a compatible primer, conversion coating, or sealant per your system. Heavy coats hide less than you think; the flatness you see now is what you’ll see after topcoat.

Special cases

Thin sheet (auto panels): Favor hand blocks sooner and lighter pressure. Heat warps panels; grind in short bursts and cool often. Your ladder may start at 80 rather than 60 to reduce risk, then 120 → 220.

Structural fillet you’re not fully flush-grinding: If the design keeps a slight crown, dress with a file until uniform, then run 120 → 220 on a hard backer to unify sheen without thinning the toe of the weld.

Inside corners & gussets: Use a safe-edge file to set flats, then short strokes with a tiny hard backer. Orthogonal scratches (straight, then diagonal) reveal leftover highs without gouging.

Mill scale adjacent to the weld: Strip scale first (grinder, needle scaler, or chemical) before your 60 step; scale is harder than the bead and will steal your stroke, causing uneven cut and undercut risk.

Aluminum & stainless (for reference): Different metals load and gall; this workflow is for mild steel. On aluminum, switch to non-loading abrasives and finer ladders; on stainless, watch heat and start at 80–120.

Pro tips

• Hard backing wins. Blocks bridge highs and keep planes true; foam and fingertips carve trenches alongside the bead.
• Let color tell the truth. Re-ink witness color after each grit; islands show exactly where to stop.
• One direction per grit. 60 straight, 120 gentle diagonal (then straight), 220 straight—leftover lines become obvious and removable.
• Change sheets early. Loaded paper skates and polishes, which makes zebra bands you’ll chase under primer.
• Stay cool. If the steel feels hot, pause. Heat softens flap discs and encourages undercut at the toes.
• Mind edges and holes. Keep blocks fully supported around slots/holes; unsupported paper “dives” and leaves crescents.

Aftercare

• Prime or convert within hours to avoid flash rust.
• If parts sit, scuff the film (not bare steel) lightly at 320 before topcoat.
• For exterior steel, seal seams after primer to prevent moisture wicking along the weld.
• Document your recipe (grits, stroke directions, primer brand, flash times) so future repairs match.

FAQs

• Why not finish with 400 on bare steel? Finer grits can burnish and reduce mechanical key. Stop at 220 on bare steel; go finer on cured primer if you’re chasing show-car smooth.
• Can I do all of this with a flap disc? You can get close, but hand blocks make the joint truly planar. Flap discs follow waves and tend to undercut next to the bead.
• What if I’ve already undercut? Stop grinding. Build the low with weld, dress with a file, then re-enter the 60 → 120 → 220 ladder on a hard block.
• Wet or dry? Dry is safer on bare steel; water invites flash rust. If you must wet-sand, dry and protect immediately.

Video

Closing

Seamless welds aren’t magic—they’re discipline. Reduce the crown with control, then trust the hard-backed ladder: 60 → 120 → 220. Keep strokes straight, pressure light, and sheets fresh, and let witness color and raking light call the stops. Do that, and your welds will vanish without trenches or zebra bands—ready for primer and a finish that looks intentionally smooth, not ground-to-death.