Blend TIG Welds on Stainless: Flap Wheels vs Fiber Discs

Stainless TIG welds can look sculptural up close—and distracting from a step back. The goal of blending is to make the joint disappear into the parent metal with a consistent brush or satin, without overheating or gouging the plate. You’ll often reach first for a flap wheel or a fiber disc on an angle grinder, but the pro result comes when you follow those tools with disciplined, hard-backed sheet sanding that flattens, aligns, and refines the scratch. This guide compares flap wheels vs fiber discs for the early knockdown and then shows a tight, reliable sheet-sanding sequence to deliver a uniform, paint- or polish-ready finish.

Why the right sanding sequence matters on stainless

Stainless work-hardens and heats fast. Coarse grinder media cut quickly but love to dig beside the bead (undercut) and leave grooves that telegraph under light. Fiber discs (resin-bonded on a stiff backing) cut flatter than flap wheels, which conform and can dish lows—yet flaps shine on curves. Either way, you’ll get the flattest, most uniform finish by switching early to hard-backed sheets and keeping strokes straight so the block bridges bead and base metal together.

Tools & supplies

• Angle grinder (variable speed preferred)
• Flap wheels (80–120) for quick, conforming knockdown on curves
• Fiber discs + rigid backup pad (80–120) for flat, controlled knockdown
• Hard sanding blocks (phenolic/steel) in long and narrow sizes
• Silicon carbide sheets for refinement: 120, 180, 320 (400+ if polishing later)
• Raking light, straightedge, and guide coat (powder or layout dye)
• Masking tape for edges, holes, and nearby features
• Solvent cleaner (wax & grease remover), lint-free wipes, vacuum/air
• PPE: respirator, gloves, eye/ear protection—stainless dust is real dust

Grit sequence

• Knockdown (grinder): 80–120 flap wheel or fiber disc, used lightly.
• Flat & unify (block sanding): 120—bridge bead and plate together.
• Refine: 180—erase 120 lines, keep planes true.
• Finish scratch (satin/brush): 320—tight, uniform matte ready for primer, clear, or brushed stainless.

Step-by-step

1. Clean, map, and protect. Degrease the weld and surrounding plate. Mask 1–2 mm shy of edges and holes so you don’t round them while blending. Dust a guide coat; under raking light it highlights grinder marks and low troughs.
2. Pick the right knockdown tool. On flats, a fiber disc on a rigid pad stays plane-true. For curves and radii, a flap wheel’s conformity helps avoid flats. In both cases, run light pressure, keep the tool moving, and stop before you’re flush—the block will set the final plane.
3. Control heat. Stainless blues fast. If the surface warms, pause. Heat expands panels, smears scratches, and can sensitize certain grades. Work cool and methodical.
4. Switch to hard-backed 120 to establish flatness. Wrap a fresh sheet around a rigid block and sand in straight, overlapping strokes that run across the weld so the block bridges bead and base metal together. Replace paper as soon as the cut slows. For predictable, consistent cut at this stage, reach for 120 Grit Sandpaper (25-pack) and keep several pieces ready.
5. Read the plane—don’t chase lows. Squeegee dust and check the guide coat. If you see a narrow dyed trough beside the bead (grinder undercut), take two or three full-width passes with the block; distributed pressure narrows troughs without digging divots.
6. Refine to 180 for uniform tooth. Change stroke direction slightly (gentle diagonal) so remaining 120 lines are obvious. Keep the block flat and pressure feather-light. For multi-panel jobs, stay stocked with 180 Grit Sandpaper (50-pack) so you never push a dull sheet that skates.
7. Blend transitions and features. Use a narrow hard backer around bolt holes, gussets, and hems. Keep strokes straight; avoid polishing arcs that print under light.
8. Set the final brush at 320. One or two even, straight passes with 320 Grit Sandpaper (100-pack) tighten the scratch into a consistent satin. Align the final strokes with the grain direction you want to keep.
9. Choose your finish path. For a brushed stainless look, stop at 320 (or 400) and clean thoroughly. For paint, epoxy primer buries a clean 180–320 scratch; build in thin coats. For mirror polish, continue 400→600→1000+ and switch to compounds.
10. Final clean and inspect. Vacuum or blow down (dry air), solvent-wipe per your coating system, and inspect under raking light. Uniform matte with no bands = ready to finish.

Flap wheels vs fiber discs—when to use which

Flap wheels: Excellent on tube, inside radii, and contoured brackets. They’re forgiving but can “track” and dish if you linger. Use light pressure and broad sweeps.

Fiber discs: With a stiff pad, they keep flats truly flat and leave a linear scratch that erases quickly with sheets. Keep the grinder square so you don’t plow a trench beside the bead.

Hybrid approach: Rough flats with a fiber disc, switch to a small flap wheel for corners, then finish everything with hard-backed sheets so the final texture matches.

Special cases

Thin automotive skins: Favor the block early and accept slower removal to protect shape. If the guide coat barely moves at 120, persist rather than leaning on the grinder.

Tubes and handrails: After flap-wheel knockdown, use a curved, hard backer (split dowel) wrapped with 180→320 to keep the arc true. Final strokes should follow the rail’s length.

Food-service stainless: Avoid contamination. Dedicate abrasives to stainless only; don’t cross-grind after carbon steel. Clean thoroughly before service.

Visible grinder undercut: You can’t sand a trench away. TIG topping or a compatible metal-filled glaze may be needed; then re-enter at 120→180→320 to blend.

Pro tips

• Hard backing wins. Foam and fingertips create scallops that catch light.
• One direction per grit: straight at 120, gentle diagonal at 180, straight again at 320.
• Guide coat = truth. Re-dust between grits and stop the moment witness color disappears evenly.
• Change sheets early. A loaded sheet skates and polishes; fresh paper cuts cooler and flatter.
• Work cool. If the panel feels warm, pause.
• Mask smart. Tape edges during coarse steps; remove for a single light pass at the finishing grit to unify sheen.

Aftercare

• Prime within the window after sanding to avoid re-contamination.
• For brushed finishes, clean with mild detergent; avoid chlorine bleach on 304/316.
• If micro-scratches appear later, a quick 320 pass in the brush direction refreshes the look.
• Record your recipe: tools, grits, stroke directions, and coating brand for repeat jobs.

FAQs

• Is 80 grit too aggressive? It removes peaks fast on the grinder but cuts deep. Follow quickly with 120 sheet sanding.
• Can I start sheets at 150? Yes, if the weld is nearly flush. If 150 skates, drop briefly to 120, then re-enter at 180.
• Do I need to go finer than 320? Only for a polished look or thin coatings. 320 sets a durable satin and primes well.
• Wet or dry? Dry is fine; if you wet-sand, dry thoroughly and avoid trapping water in seams.
• Why are dark streaks appearing? Likely heat tint or contaminated media. Cool the work, switch to stainless-only abrasives, and re-sand lightly with 180→320.

Video

Closing

Flap wheels and fiber discs both have a place—use them to get close. The flat, uniform finish that hides under light comes from disciplined sheet sanding on a hard backer: 120 → 180 → 320 with straight strokes, fresh paper, and a cool workpiece. Follow this plan and your stainless TIG joints will melt into the parent metal, whether you’re leaving a classy brush, priming for paint, or stepping toward polish.