Surfboard Hot-Coat Sanding: 120 to Polish (No Flat Spots)

The hot coat (fill coat) levels weave and locks in your lamination. To get that glassy finish without waves, you need a sanding plan that flattens drips and texture, tightens the scratch, and sets you up for either a gloss coat or a full polish. Go too coarse and you’ll trench; too fine too soon and you’ll only polish highs. This guide gives you a disciplined, hard-backed sequence—from 120 → 240 → 320 → 400 → 600 → 1000 → 1500/2000—that keeps decks true, rails crisp, and logos safe.

Why sanding matters on fiberglass/epoxy hot coats

Epoxy and polyester hot coats are films over fabric. Random pressure on a soft pad follows waves, rounds rails, and can kiss through to cloth on high spots. A hard backer bridges highs and lows, shearing peaks into the plane instead of digging troughs. Tight grit jumps prevent “telegraphing” where coarse arcs show under gloss. Work cool, swap paper early, and let raking light, not guesswork, tell you when to stop at each step.

Tools

• Hard sanding blocks: long flat for decks/bottoms, narrow block for stringers, tiny detail block for fin boxes and noses/tails
• Optional DA/RO with a firm interface pad (finish each grit by hand on a hard backer)
• Silicon carbide sheets: 120, 240, 320, 400, 600, 1000, 1500 (2000 optional)
• Raking light, soft pencil for witness marks, and a very light dry guide coat
• Masking tape for rails, logos, pinlines, fin boxes, and leash plugs
• Vacuum with brush tip, microfiber towels, and a rubber squeegee
• Spray bottle with clean water + a drop of dish soap (for 600+)
• PPE: respirator, eye/ear protection, gloves

Grit sequence

• Flatten & unify: 120 grit on a hard backer to take down nibs, sags, and RO peel flat.
• Refine & tighten: 240 → 320 to erase 120 lines and even the field.
• Pre-gloss prep: 400 → 600 to leave a shallow, uniform matte that vanishes under gloss or is the base for polish.
• Polish-ready: 1000 → 1500/2000 wet for a fast, cool buff if you’re skipping a sprayed gloss.

Step-by-step

1. Confirm cure and clean. Sand only after the hot coat is fully cured (per your system). Wash off amine blush on epoxies before sanding—it clogs paper and causes fish-eyes later. Vacuum and wipe the board clean.
2. Mask rails, boxes, and art. Double up tape on crisp rails and around fin boxes/leash plugs. Protect logos and pinlines with a tape frame so coarse grits don’t scuff the ink.
3. Map truth with witness marks. Under raking light, lightly pencil a grid and dust a faint guide coat. Color on peaks should vanish first; islands that linger in lows tell you to keep strokes full-length, not to dig locally.
4. Open at 120 on a hard block. Wrap fresh 120 and sand in long, straight, overlapping strokes along the board’s length. Keep pressure feather-light; no fingertip spot-sanding. Swap paper the instant the cut slows—dull sheets polish highs. For a consistent opener you can change early instead of pressing harder, keep 150 Grit Sandpaper (25-pack) at hand if your hot coat is already fairly level, or test 120 on a scrap before committing.
5. Read, don’t chase lows. Squeegee the slurry/dust and inspect. When only tiny guide freckles remain in the bottoms, stop the coarse step. Chasing the very last low creates dishes that telegraph under gloss.
6. Refine to 240 for uniform tooth. Change stroke angle slightly (gentle diagonal) so leftover 120 tracks stand out, then return to straight strokes. The board should now read as a calm, uniform matte with no shiny islands. Stock mid-quantity sheets so every pass cuts cool and predictable—something like 240 Grit Sandpaper (50-pack) keeps the tempo steady across a run of boards.
7. Set your pre-gloss scratch at 320 → 400 → 600. One or two even passes per grit, keeping strokes straight and the block fully supported across stringers and rails. If you’re spraying a gloss coat, 320–400 is usually enough; for a sanded finish, take it to 600 so the micro-texture reads satin, not scratchy.
8. Optional wet steps at 600. If allowed by your system, switch to a light soapy mist at 600 to keep temps down and prevent loading. Keep water off raw foam or unsealed edges.
9. Polish route: 1000 → 1500/2000. If you’re skipping a sprayed gloss, move to 1000 wet with a hard backer, then 1500 (and 2000 on dark pigments). Keep strokes straight and panels cool. Consistency here is everything—finishing with a ready supply like 1000 Grit Sandpaper (100-pack) makes the buff fast and even.
10. De-dust and preview. Vacuum, wipe, and do a solvent wipe (compatible with your resin) to “wet” the surface visually. Uniform haze with no bands means you’re ready to gloss or buff.

Special cases

Stringer ridges: Wood sands slower than resin. Keep the block long and strokes full-length; do not tip into foam shoulders. If the stringer stands proud, take two extra full-length passes at the current grit—never fingertip just the wood.
Cloth kisses (weave ghosting): If you nick the lamination, stop sanding. Spot-coat resin, level at 320 after cure, and then re-enter your sequence. Don’t try to hide weave with finer grits; you’ll only polish the problem.
Rail safety: Leave two layers of tape on rails through 320. Remove the top layer and make one feather-light pass parallel to the edge at 400–600 with a worn sheet to blend sheen without thinning.

Pro tips

• Hard backing wins. Blocks bridge highs and keep decks true; foam pads carve dishes that glare in raking light.
• One direction per grit. 120 straight, 240 gentle diagonal (then straight), 320+ straight. Leftover scratches become obvious—and removable.
• Change sheets early. Loaded paper skates and heats resin, making shiny bands that stubbornly reappear.
• Work cool & clean. If the panel feels warm, pause. Vacuum between grits; stray 120 grains at 600 make mystery lines.
• Trust the light. Read the board under raking light from nose and tail. What you miss now will shout after gloss.

Aftercare

• If you glossed: Let the coat cure hard, then denib the film lightly (1000–1500) before final buff.
• If you stopped at sanded finish: Rinse thoroughly to remove slurry, then wipe dry. Avoid harsh cleaners in the first week.
• Store boards flat or nose-down while films cure.
• Log your exact recipe (grits, stroke directions, resin brand, cure times) so your next build matches.

FAQs

• Can I start at 80 to go faster? Only if the hot coat is very lumpy—and only on a long, hard backer. Expect to spend more time refining and a higher risk of telegraphing.
• DA or hand block? A DA with a firm pad speeds flats, but finish each grit by hand on a hard backer for shallow, linear scratches.
• Wet or dry? Dry through 400; a light soapy mist is great at 600+ for heat control. Keep liquids away from unsealed foam.
• Why do zebra bands show after gloss? You likely polished highs with dull paper or a soft pad. Re-enter one grit coarser on a hard block, make two full-length passes, and re-tighten the sequence.

Video

Closing

Flat, fast, glassy results come from control—not force. Keep the backing hard, pressure light, and the grit ladder tight: 120 → 240 → 320 → 400 → 600 → 1000 → 1500/2000. Change sheets early, work under raking light, and stop the moment the surface reads evenly. Follow this playbook and your hot coat sets the stage for a mirror gloss or a clean satin polish—no flat spots, no weave ghosts, just smooth, speed-ready curves.