Blend TIG Welds on Stainless: Flap Wheels vs. Fiber Discs

When you’ve laid down a clean TIG bead on stainless but need it to disappear under a uniform brushed or paint-ready surface, the right abrasive sequence is everything. Stainless is springy and work-hardens quickly; if you attack it with the wrong tool, you’ll dig a trough beside the bead, overheat the panel, or leave deep tracks that refuse to hide. This guide explains when to reach for flap wheels, when fiber discs on a firm backing are smarter, and how to finish the job with sheet abrasives so the surface is flat, smooth, and consistent.

Why sanding (and tool choice) matters

Flap wheels remove material quickly and conform to contours, but that same flexibility can “dish” soft zones around a weld. Fiber discs mounted on a rigid backing plate cut flatter, keeping the whole area in a single plane—ideal for blending a bead flush without hollowing the heat-affected zone (HAZ). No matter which powered step you use first, you’ll still need disciplined, backed sheet sanding to erase coarse swirls and set the final scratch that matches your finish target (brushed stainless or primer-ready). Controlled pressure, smart grits, and frequent inspections under a raking light prevent mistakes that show up only after the last pass.

Tools

• Angle grinder (variable speed preferred) with flap wheels and fiber discs + firm backing plate
• Hard sanding block (phenolic/steel) sized to the panel; small precision blocks for tight spots
• Silicon carbide wet/dry sandpaper sheets: 120, 240, 400 grits
• Straightedge, marker/layout dye for witness coats, and a bright raking light
• Masking tape to protect crisp breaks and features
• Shop vacuum, clean rags, and metal-safe degreaser
• Eye, hearing, and hand protection; spark-safe clothing

Grit sequence (stainless TIG blend)

• Cut/shape: Flap wheel or fiber disc in the 60–80 range to knock the crown (use fiber + firm plate when flatness matters most).
• True and refine: Backed sheet sanding at 120 to unify the plane and erase heavy spiral marks.
• Blend: 240 grit to smooth the HAZ and set a uniform hand-sanded texture.
• Finish: 400 grit for paint-ready surfaces, or stop at 240 for a coarser shop-brush look; follow with Scotch-Brite if a linear brush is desired.

Step-by-step

1. Protect and map the work zone. Mask crisp edges, body lines, and features you don’t want to round over. Black out the weld and surrounding area with marker or layout dye—this witness coat tells you what’s still high or low as you sand.
2. Choose your power step: flap or fiber. If you’re on a curved guard or a tube, a flap wheel’s conformability is an asset. On panels or flat covers, a fiber disc on a firm backing plate keeps the whole area in plane. Keep speed moderate and pressure light; stainless heats quickly, and heat grows the HAZ.
3. Knock the crown, then stop. Glide across the bead with long, overlapping strokes. Your aim is “near flush,” not perfectly flush yet. The moment the dye shows you’re touching the surrounding field, shift away from the wheel; the rest belongs to the block and sheets.
4. True the plane at 120 on a hard block. Wrap a fresh 120-grit sheet on a rigid block and sand in straight, overlapping passes that span the bead and the adjacent stainless together. Re-ink the area as needed and sand until the color disappears evenly—your signal that you’re truly flat. Stock reliable sheets like 120 Grit Sandpaper (25-pack) so you can swap the moment the cut slows.
5. Blend the HAZ at 240 without changing planes. Switch to 240 and alter your stroke direction slightly (e.g., diagonal instead of straight) so any remaining 120 lines stand out. Keep the block fully supported; don’t tilt into edges. For sustained runs—rails, guards, long seams—minimize downtime with 240 Grit Sandpaper (50-pack) so every station has fresh sheets ready.
6. Chase consistency, not lows. If you see a faint low beside the weld, resist poking at it with fingertips or narrow blocks. Widen your passes so pressure rides the whole region; two light, full-width sets beat one aggressive localized cut.
7. Set the finish at 400. Make a final, even pass with 400 in the direction you want the grain to read (or cross-hatch once, then finish with the grain). This shallow, uniform scratch covers easily under primer and looks clean under a Scotch-Brite linear brush. For panel batches, keep consistency with 400 Grit Sandpaper (100-pack).
8. Clean and inspect. Vacuum grinding dust, wipe with a stainless-safe degreaser, and inspect under raking light. You should see a uniform, straight scratch, no spirals, no shiny islands, and—critically—no trough shadow along the bead line.
9. Optional: Brush the grain. If you want a linear brushed look, follow 240 or 400 with a non-woven medium pad in straight, aligned strokes. Keep pressure light to avoid wavy banding.
10. Prep for coating (if painting). After degreasing, prime soon (epoxy or etch per system). Avoid heavy, single coats—build gradually, then scuff 320–400 before color.

Special cases

Thin sheet or heat-sensitive parts: Favor fiber discs with a firm plate to limit local heat and use short bursts. Immediately switch to 120-backed sheets to finish the last fractions of a millimeter. If distortion appears, stop cutting metal and plan to restore flatness with filler after primer.

Inside corners and fillets: Make a custom backer (e.g., a ground-square steel bar) and wrap sheets tightly. Work from both directions to avoid rolling the corner open.

Tubes and handrails: Flap wheels shine here—but finish with backed sheets to keep the arc true. Mark longitudinal stripes with marker and sand until they fade evenly to avoid flats.

Contamination: Stainless shows every fingerprint and oil smear; degrease before abrasive steps so you don’t rub contaminants into scratches. Between stages, wipe clean so you’re not dragging old swarf across new scratches.

Matching an existing factory brush: Determine the direction and coarseness by test passes on scrap. Many brushed finishes read close to a 240–320 linear scratch; do a small, taped-off test area first.

Pro tips

• Keep pressure light and even; let the grit do the work. Heavy hands tilt the block and dish the HAZ.
• Refresh paper early. Dull sheets skate and generate heat, which grows the HAZ and slows you down.
• Use witness coats (marker or layout dye). Sand until color disappears evenly; isolated color means lows you shouldn’t chase aggressively.
• Don’t overstay the power step. Wheels and discs remove fast—but they also create problems fast.
• Finish with straight strokes. Random swirls are hard to hide; linear scratches telegraph as an intentional grain.
• Mind sparks and dust. Stainless grinding throws hot debris; keep flammables away and wear proper PPE.

Aftercare

• Degrease before any primer or topcoat; avoid chloride-based cleaners that can stain stainless.
• If left bare, finish with a uniform linear brush and clean periodically with a stainless-safe spray.
• For outdoor parts, consider passivation after heavy sanding to restore corrosion resistance.
• Handle with gloves after prep—skin oils can print and interfere with paint adhesion.

FAQs

• Flap wheel or fiber disc—which is better? For flat work, fiber + firm plate keeps panels true. For curves and tubes, flap wheels conform better. In both cases, finish with backed sheets for consistency.
• Why not finish entirely with the grinder? Spiral grinder tracks are hard to erase and often leave shallow waves. A hard-backed sheet step trues the surface and sets a clean final scratch.
• Is wet sanding OK on stainless? It’s possible, but most pros dry-sand for visibility and to avoid trapping moisture under primer. If you do wet sand, dry thoroughly and dewater seams before coating.
• What if I see a faint valley beside the bead? Stop the wheel work, widen your blocked sheet passes, and blend the area as a unit. Don’t dig at the low—restore flatness gradually.
• What finish scratch hides best under paint? A uniform 400 line covers easily with quality primers; scuff 320–400 again after primer surfacer before color for show finishes.

Video

Closing

Blend stainless TIG welds like a pro by using each abrasive where it excels: flap wheels for curves, fiber discs for flat control, and hard-backed sheets to true the plane and dial in the final scratch. Keep pressure light, refresh paper early, and inspect under raking light. With a 120→240→400 progression on a rigid block, you’ll get flat, consistent results that brush beautifully or hide perfectly under primer.