Surfboard Hot Coat Sanding: 120 Grit to Polish, Step-by-Step

The hot coat is where a surfboard goes from rough glass to sleek hydrodynamics. Done well, sanding the hot coat flattens ripples, removes laps and drips, and sets you up for a glossy, durable finish that slices through water. Done poorly, it leaves low spots, burn-throughs into the lamination, and swirl marks that telegraph under polish. This guide lays out a clean, repeatable workflow—from the first leveling passes at 120 grit to a polish-ready surface—so you get fast water and a pro look without the headaches.

Why sanding the hot coat matters

That glossy resin skin isn’t just cosmetic. A flat, fair hot coat reduces drag, sheds water evenly, and makes your rail transitions feel predictable at speed. The catch is that polyester or epoxy resin is hard, yet thin in places. If you push too coarse, you’ll cut grooves that take forever to erase. If you chase lows with fingertip pressure, you’ll dish soft areas or burn through to glass fibers—an instant repair. A disciplined grit ladder with hard backing, light pressure, and frequent checks under raking light keeps the surface fair while protecting the lam.

Tools & supplies

• Rigid sanding block (phenolic/cork) for flats; shaped mini-blocks for rails and concaves
• Random-orbital sander with a firm interface pad (optional; hand blocking controls flatness best)
• Silicon carbide wet/dry sheets: 120, 220, 320, 400, 600, 800, 1000, 1500
• Spray bottle with clean water + a drop of dish soap (lubrication)
• Squeegee and microfiber towels
• Grease pencil for witness marks and trouble spots
• Masking tape to guard leash plug, fin boxes, logos, and the stringer if needed
• Bright raking light and a straightedge
• Respirator, eye protection, and gloves

Recommended grit sequence (hot coat → polish)

• Level: 120 → 220 to knock down highs, laps, and drips quickly while staying controllable.
• Refine & fair: 320 → 400 → 600 to erase coarse tracks and bring the board to a uniform matte.
• Pre-polish: 800 → 1000 → 1500 to leave a tight, shallow scratch that compounds out fast.

Step-by-step

1. Mask critical areas and plan your zones. Tape around leash cups, fin boxes, vents, and any decals you want protected. Divide the board mentally into bottom flats, rails, and deck. Work one zone at a time so you don’t lose track of scratch removal.
2. Map highs and lows under raking light. Mark drips, lap lines, and shiny low islands with a grease pencil. A few light cross-hatch squiggles across a section act as witness marks—sand until these just disappear evenly.
3. Start leveling at 120 grit (wet). With a hard block, mist the surface and sand in long, overlapping strokes that follow the board’s length. Keep pressure even and the block flat—no fingertip poking. Swap sheets the instant the cut slows to avoid dragging scratches. For consistent, flat cut during this first critical stage, stock 120 Grit Sandpaper (25-pack) and change frequently.
4. Check, squeegee, and correct your plane. Squeegee the slurry, wipe dry, and sight with the light. Shiny islands mean lows—resist digging. Widen your strokes so pressure spans highs and lows together. If you see weave print-through anyplace, stop and plan a spot repair before you continue.
5. Refine to 220 grit. Re-mark a faint grid and sand at 220 to erase 120 lines and remove the last of the laps and drips. Stay methodical—rails last. On stringers, reduce pressure and keep the block centered; wood cuts slower than resin and can stand proud if you ride the foam too hard.
6. Fair the surface at 320, then 400. These are your “make it look even” stages. Slightly change stroke direction (e.g., diagonal at 320, lengthwise at 400) so remaining scratches stand out. Keep the pad flat. Rails: use a shaped block that matches the curvature so you don’t flatten the tuck.
7. Set uniform tooth at 600 grit. A good 600 pass makes the board evenly matte with no coarse lines. It’s also a sweet stopping point if you plan to spray a clear or hot-coat another thin pass. For efficient batch work (multiple boards) without mid-session shortages, keep 600 Grit Sandpaper (50-pack) at the ready so every zone gets a fresh, fast-cutting sheet.
8. Pre-polish with 800 and 1000. Mist lightly and sand in straight, overlapping strokes. On concaves, roll the block through the curve but avoid point pressure at the apex. Your aim is a tight, uniform haze—no shiny patches, no directional bands.
9. Final hand pass at 1500. This step pays off at the buffer. Make one light, straight-line pass until the surface reads as an even, ultra-fine matte. For large boards or production runs, finishing this stage with 1500 Grit Sandpaper (100-pack) keeps the cut consistent from nose to tail.
10. Clean thoroughly before compound. Rinse or wipe away all slurry and dust. Any abrasive left on the pad becomes a rogue scratch maker. Dry completely—water hiding at fin boxes or plugs will splash under the buffer.
11. Polish with discipline. Use a firm foam pad and a non-aggressive compound rated for 1500-prep surfaces. Keep speed moderate, pad flat, and heat low. Work small sections, wipe clean, and step to a finer polish if needed. If you see trails, back up: a few more 1500 strokes usually erase them faster than chasing with compound alone.

Special cases

Epoxy vs polyester hot coats: Epoxy often feels “rubbery” and loads paper sooner—change sheets early and keep water flowing. Polyester cuts faster but can be thinner near rails; watch for burn-through.

Stringer proud after leveling: The wood can stand high as surrounding resin drops. Kiss the stringer lightly with 320–400 on a narrow block, then blend the foam side very lightly to avoid a flat next to the seam.

Weave print-through: If you’ve revealed glass texture, don’t polish it and hope—spot fill with resin, re-level at 320–400, then continue the sequence.

Rails and tucked edges: Shape a mini block to match the rail radius. Finish rails by hand with lengthwise strokes; cross-grain arcs are what create flat spots you’ll feel under your palm.

Glassed-on fins: Tape the fin faces and base fillets tightly. Work up to the tape line and remove the tape for the last light pass to blend.

Logo windows and pinlines: Treat these like edges—minimal pressure, fresh paper, and straight strokes to avoid distortion.

Pro tips

• Hard backing wins. Random orbital is fast, but a rigid block keeps the plane true—especially on big bottom flats and decks.
• One direction per grit. Alternate directions between grits so you can see when an old pattern is gone. Finish 1500 in straight nose-to-tail lines for the cleanest buff-out.
• Change sheets early. Loaded paper drags scratches across shiny low spots; fresh sheets cut cooler and straighter.
• Keep water minimal but present. A light soapy mist is enough. Floods hide lows and can hydroplane the sheet.
• Read the light. A raking light at 30–45° tells you truth—uniform matte means you’re done with that grit.
• Don’t chase a single low. Widen your strokes and bring the whole zone down together, or spot-fill and re-level later.

Aftercare

• Rinse the board thoroughly after polishing and remove compound from fin boxes, leash cups, and vents.
• Let the board sit out of direct sun to finish outgassing, especially with fresh epoxy.
• For a gloss that lasts, avoid harsh cleaners; use mild soap and water and a soft cloth.
• Small scuffs later? Hand-rub with 1500 and a touch of polish instead of jumping straight to aggressive compound.

FAQs

• Can I start at 80 or 100 to go faster? Not recommended on hot coats—it’s too easy to cut a trench you’ll chase for hours. 120 balances speed and control.
• Dry or wet sand? Wet keeps the sheet cooler and reduces dust; use a light mist so you can still read the surface. Dry is fine for quick spot work if you vacuum often.
• What if I see micro pinholes after 400? Clean, spot-fill with resin, block level at 400–600, and re-enter the sequence.
• Do I have to go to 1500 before polish? You can compound from 1000, but 1500 shortens buff time and reduces swirl risk.
• How do I keep rails from going flat? Use a shaped block, minimal pressure, and lengthwise strokes; never “rock” a flat block around the curve.

Video

Closing

A fast board is a fair board. Work methodically: level at 120, refine through 220→400→600, then pre-polish 800→1000→1500. Keep the block hard, your pressure light, the water just enough, and your sheets fresh. Follow this sequence and you’ll land a slick, durable finish that paddles clean, planes early, and looks as good as it rides.