Blend TIG Welds on Stainless: Flap vs Fiber Discs (120–320)

TIG on stainless can look like a perfect stack of dimes—until the light hits the panel and a proud ridge, heat tint, or grinder arc gives the repair away. The goal when blending TIG welds isn’t just removal; it’s to flatten the crown, erase discoloration, and leave a controlled satin that matches the surrounding finish—without undercutting, overheating, or contamination. This guide compares flap discs and fiber discs for the machine step, then shows how to finish correctly by hand on a hard block so the blend reads seamless in any light.

Why tool choice and grit discipline matter on stainless

Stainless work-hardens, heats quickly, and shows every scratch. Open-coat flap discs conform and are forgiving, but they can dig a shallow trough if you lean on the edge. Fiber discs (resin-bonded on a stiff backing) cut flatter and cooler on a hard pad but demand control. Either way, don’t “finish on the machine.” Switch to hard-backed sheets in a tight ladder—typically 120 → 180 → 320—to align the scratch, keep the plane honest, and match common #4 brushed sheens without halos or trenches.

Tools & supplies

• Angle grinder (variable speed preferred) and flat flap discs (80–120) for initial crown reduction
• Angle grinder with a hard backing pad and fiber discs (80–120) for flat cutting on larger flats
• Hard sanding blocks: long flat block for panels, small rigid block for flanges/returns, tiny detail block
• Silicon carbide sheets: 120, 180, 320
• Raking light, straightedge, and layout dye or marker as witness color
• Files (safe-edge) for corners and holes
• Clean water for a light lube at 320 if desired
• Microfiber towels, vacuum with brush tip, stainless-safe panel wipe
• PPE: eye/ear protection, gloves, respirator

Recommended grit sequence (machine → hand)

• Machine reduction: 80–120 flap or fiber disc to knock down the weld crown without gouging shoulders.
• Hand sequence on a hard block: 120 → 180 → 320 for a uniform satin that matches production finishes.

Step-by-step

1. Prep and protect. De-burr spatter and clean weld soot. Mask adjacent finished faces and any brushed grain you need to preserve. Apply witness color (layout dye or marker) across the bead and 1–2 inches beyond; it shows exactly what you’re removing.
2. Reduce the crown with control. For curved parts or inside corners, a flat flap disc is forgiving—keep the disc flat to the work, never on the edge, and use light, short passes. On larger flats, a fiber disc on a hard pad keeps the cut planar. Stop while the bead is just barely proud; the block will finish the flattening.
3. Open at 120 on a hard block. Wrap a fresh 120 sheet around a rigid block and sand with straight, overlapping strokes that bridge weld and parent metal. Replace paper as soon as the cut slows—dull sheets skate and polish stainless. It helps to stage reliable stock so you never press harder; keep 120 Grit Sandpaper (25-pack) ready at the bench.
4. Read the plane; don’t chase lows. Wipe or vacuum and inspect under raking light with a straightedge. Witness color should disappear first on the crown, then evenly from the field. If tiny islands remain in micro-lows, take two full-width passes at 120—never fingertip-dig, which creates a trench you’ll see after finish.
5. Refine to 180 across the zone. Change stroke angle slightly (a gentle diagonal), then return to lengthwise strokes for the last passes. This erases 120 tracks and begins to unify the sheen. For longer seams and batch work, keep mid-quantity sheets on hand so every pass cuts cool and predictable—something like 180 Grit Sandpaper (50-pack) prevents the “just one more pass” trap with a dull sheet.
6. Set the satin base at 320. One or two even passes tighten the 180 pattern into a uniform, directional matte that blends with common #4 brushed finishes. Keep pressure feather-light and the block fully supported. For consistent end-to-end results, finish with fresh stock such as 320 Grit Sandpaper (100-pack) so the last panel sands like the first.
7. Optional: fine-tune the grain. If the surrounding stainless has a pronounced factory brush, use a worn 320 on a narrow hard backer to pull long, uninterrupted strokes in the original grain direction. Avoid tight circles—those read as halos in sunlight.
8. De-dust, de-tint, and clean. Vacuum thoroughly. If you used dye, wipe with a stainless-safe panel wipe so no color remains in pores. Oils from handling can create splotchy sheen; clean until a mist of panel wipe wets the surface evenly without beading.

Special cases

Thin sheet & heat control: Stainless moves with heat. Use light pressure, sharp abrasives, and short machine passes; let the part cool. If the panel warms to the touch, pause.
Curved tubes & handrails: Flap discs conform; finish by hand with a curved hard backer (split dowel) at 180→320 to keep the radius true. Don’t wrap paper around your finger; it dishes the curve.
Food-service finishes: Many kitchens spec a consistent #4 (≈150–180) or finer. Stop at 180 for a closer match, then pull long strokes with 320 only if needed to equalize sheen. Clean with non-chloride cleaners.
After pickling/passivation: If you chemically removed heat tint, neutralize and rinse thoroughly. Your abrasive steps remain the same, but expect slightly faster cutting on the freshly de-oxidized surface.
Grain-matched panels: Before you sand, photograph the grain direction and stroke length. Recreate that rhythm at 320 with a long backer to avoid a “repaired patch” look.

Pro tips

• Hard backing wins. Blocks bridge highs and keep planes true; foam pads and fingertips carve shallow troughs that glare after finish.
• One direction per grit. 120 straight, 180 gentle diagonal (then straight), 320 straight. Leftover lines become obvious—and removable.
• Change sheets early. A loaded sheet skates and polishes; fresh paper cuts cooler and flatter, especially at 320 where sheen consistency matters.
• Stay clean. Cross-contamination (carbon steel dust on stainless) causes rust blooms later. Dedicate abrasives for stainless and wipe tools before you start.
• Match the environment. Note the final brush direction/width on scrap before the part.

Aftercare

• Rinse or panel-wipe to remove residues, then handle with clean gloves to avoid fingerprints on fresh satin.
• If the part will be painted, apply the system’s stainless-compatible primer promptly; for bare stainless, consider a final passivation step per spec.
• For kitchens or handrails, clean with non-chloride detergents; wipe with the grain to maintain the uniform brush.

FAQs

• Flap disc or fiber disc? Flap discs are forgiving on curves; fiber discs on a hard pad cut flatter on big panels. Either way, finish by hand on a hard block for a truly planar blend.
• Start at 80 by hand? Only if the crown is high; otherwise open at 120.
• Do I need to go finer than 320? For a brighter satin, 400–600 is fine—but 320 matches common #4 brush. Go finer only if the surrounding finish demands it.
• Wet or dry? Dry through 180; a light mist at 320 is OK—don’t flood.
• What if I see a shallow trench along the weld? That’s undercut from edging a wheel or fingertip sanding. Re-ink, then take two full-width passes at 120 on a long, hard block to rebuild a flat, then re-refine.

Video

Closing

Stainless rewards control, not force. Use a forgiving machine step to reduce the crown, then make the surface truly planar with a hard-backed ladder—120 → 180 → 320. Keep strokes straight, change sheets early, and let witness color and raking light call the stops. Do that, and your TIG blends will disappear into the parent metal with a uniform satin that looks deliberate in any light.