Surfboard Hot Coat Sanding: 120 Grit to Polish (Pro Workflow)

A clean, flat hot coat is the foundation of every fast, quiet board. But sanding that glassy shell without burning through the weave, rounding rails, or leaving telegraphed scratches takes more than a random sander and hope. This guide lays out a step-by-step workflow—from first leveling at 120 to an optional polish—that keeps contours true, laps safe, and the gloss you want within reach.

Why hot-coat sanding matters

The hot coat (fill coat) seals the weave and gives you film to level. Too coarse and you’ll trench resin and kiss the cloth; too fine and you’ll just polish highs while lows stay glossy. Heat from dull paper or heavy pressure risks swirl and print-through. A tight ladder—120 → 220 → 320/400 → 600 → 1000+—on a hard backer flattens fast, then tightens the scratch so gloss or matte reads even. Surfboard contours—concaves, hips, hard rails—magnify tiny errors. If your scratch runs across the board in arcs, sunlight will show bands; if you chase one low with fingertip pressure, you’ll dish the foam/glass and create a permanent distortion. Keeping strokes long and straight nose-to-tail on a rigid backer lets pressure bridge highs and lows so the plane stays true. Guide coat and pencil marks keep you honest: stop the moment color disappears evenly.

Tools & supplies

• Hard, flat sanding block; narrow rail block
• RO sander with a firm pad (optional—finish by hand)
• Silicon carbide wet/dry: 120, 220, 320/400, 600, 1000–1500
• Masking tape for rails, stringer, fin boxes/vents
• Raking light; pencil for witness marks
• Vacuum, microfiber, plastic squeegee
• Spray bottle (water + drop of soap); rinse buckets
• PPE: respirator, eye/ear protection, gloves

Why hard backing? Resin is harder than foam and softer than glass; a rigid block bridges the micro-terrain so you shear peaks instead of carving troughs. Why silicon carbide? It cuts brittle plastics and resin cleanly and stays sharp wet. Why a firm RO pad? Soft pads follow orange peel; a firm pad keeps the sander from dishing.

Recommended grit sequence

• Level: 120 knocks down orange peel, laps, nibs.
• Refine: 220 erases 120 and trues the plane.
• Tighten: 320–400 sets even tooth.
• Pre-polish: 600–1000 leaves a fast-buff haze.
• Optional polish: 1500–3000 then compound.

Step-by-step

1. Confirm cure & prep. Hot coat must be fully hard; wash amine blush on epoxy. Mask fin boxes/vents. Tape 1–2 mm shy of rail apex and tail edges.
2. Map the surface. Under raking light, pencil a loose grid on deck and bottom, and circle known pain points—laps, plugs, hips. The pencil vanishes evenly when a zone is truly level. If you like, mist a faint guide coat; it clings in orange-peel pockets and along laps so you can see progress.
3. Open at 120—flat and cool. 120 on a hard block, long overlapping strokes nose-to-tail. Light pressure, swap sheets early. Start with fresh stock like 120 Grit Sandpaper (25-pack).
4. Bridle the rails. Use a narrow hard rail block; short strokes tangent to the rail. Keep tape on at 120. If weave ghosts appear, move on.
5. Squeegee & read. Pull slurry and check the grid. Gloss islands = lows; take two full-length passes, not fingertip digs.
6. Refine to 220. Change angle slightly to see 120 lines, then go lengthwise. RO with a firm pad is OK, but finish by hand. Stay stocked with 220 Grit Sandpaper (50-pack).
7. Rail blend at 220. Pull tape; two feather passes along the apex. If the stringer is proud, kiss it with a narrow block.
8. Tighten at 320–400. One brief, even pass until 220 disappears into a fine haze. 320 leaves tooth for a sanded-matte finish; 400 leans smoother and is ideal before gloss. Stay on a hard backer to protect contours and rail crispness.
9. Pre-polish 600–1000. Wet-sand with a light soapy mist; straight strokes. Consistency is easiest with 1000 Grit Sandpaper (100-pack).
10. Optional machine polish. From 1000–1500 scratch, use firm foam, low RPM, light pressure. Keep the pad flat and cool.
11. Clean & inspect. Rinse, dry, wipe. Fix bands or shiny islands with the previous grit on a hard backer; re-enter your final grit.
12. Edge protocol. Keep tape on through coarse steps to protect corners. For the last pass at each grit, peel the tape and make one feather-light stroke parallel to the edge to unify sheen without thinning.

Special cases

Epoxy vs polyester: Epoxy can blush—wash first. It clogs paper; change sooner and wet-sand more.
Cut-through at laps/rails: Stop, feather one grit finer, spot-fill if needed, re-sand after cure.
Stringer proud/shy: Hit a proud stringer first with a narrow block; don’t dish adjacent laminate.
Heel dents: Level globally; micro hot-coat dents, then re-sand two grits finer.
Color/glitter laminations: Coarse scratches can leave ghosts under clear. Favor the 120 → 220 → 400 path, then jump to 1000 if you plan to polish; keep pressure extra light over graphics and cutlaps.
EPS core boards: Heat is the enemy. Use more wet steps (320+), change sheets early, and never let the surface warm to the touch.
Soft-top repairs: Treat the resin patch like any hot coat but keep abrasives away from foam skins; mask aggressively and stay on a hard block.

Pro tips

• Hard backing wins; foam/fingers dig troughs.
• One direction per grit: 120 straight, 220 diagonal, 320/400 straight, 600–1000 straight wet.
• Change sheets early to avoid heat and swirl.
• Keep rails taped at coarse steps; blend lightly after.
• Guide coat tells the truth; stop when it’s gone.
• Work cool; warm resin = swirls.
• Vacuum between steps; avoid rogue grit.

Aftercare

• Rinse and dry the board; avoid salt crust near plugs.
• Matte: after cure, light protectant to even hand oils (no silicone near future repairs).
• Gloss: keep clean/cool the first week.
• Later scuffs: quick 1000→1500 pass and light polish.

FAQs

• Can I start at 80? Only on wild texture, with caution. 80 makes trenches; 120 is safer.
• RO or hand block? RO with a firm pad speeds flats; finish each grit by hand on a hard backer.
• Dry or wet? Dry through 220 (even 320/400). Wet at 600–1000 for cool, uniform haze.
• Do I need 3000? No. 1000–1500 buffs fast; 3000 just shortens compound time.
• I see weave before flat. Stop. Either film is thin or pressure is high. Spot-fill, cure, re-enter two grits finer.
• Why nose-to-tail strokes? Concaves and rocker blend that way; straight strokes bridge highs and lows and avoid cross-banding under light.
• My paper loads fast at 220. Resin dust + heat = loading. Change sheets sooner, lighten up, and consider a light wet mist from 400 upward.
• Do I need a guide coat? It’s optional but powerful. A whisper of contrasting color instantly shows untouched lows and leftover lines.
• How do I protect fin boxes? Mask the flange and stand proud of it with a hard block so you don’t round the edge; remove tape for the final light blend.

Video

Closing

Leveling a hot coat without burn-through is about restraint and sequence. Start cool at 120 on a hard backer, refine at 220, tighten at 320/400, then wet-sand 600–1000 if you’re chasing gloss. Keep rails safe, strokes straight, and sheets fresh. Do that and the board reads flat in any light, rails stay crisp, and the finish—matte or mirror—looks intentional and pro.