6 Best Sandpapers For Fiberglass Repair On Small Boats Pros Swear By

You’re staring at that spider crack in the gelcoat, or maybe the gouge on the chine from a close encounter with a dock. You know the repair is straightforward, but the difference between a patch that blends in perfectly and one that screams "I’m a repair!" often comes down to one thing: your sanding technique. Choosing the right sandpaper isn’t just a minor detail; it’s the foundation of a professional, invisible fiberglass fix.

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Understanding Grits for Fiberglass Boat Repair

Think of sandpaper grits like gears on a bike. You wouldn’t start up a steep hill in your highest gear, and you shouldn’t start a fiberglass repair with ultra-fine sandpaper. The number on the back tells you everything: a lower number means a coarser, more aggressive grit, while a higher number means a finer, smoother grit. Getting this right is crucial for efficiency and quality.

For major work, like grinding out damaged fiberglass or shaping thick polyester filler, you’ll start low, somewhere in the 36 to 80-grit range. This is for rapid material removal. Once the shape is close, you’ll move to the 80 to 180-grit range for fairing and feathering the edges of your repair into the surrounding area. This is where you create the smooth, flowing transition.

The finishing stages demand a lighter touch. Prepping for primer usually involves the 220 to 320-grit range, which is fine enough to remove the scratches from the previous steps without being too smooth for the primer to grip. For that final, glass-smooth gelcoat finish before compounding and polishing, you’ll be working your way up through 400, 600, 800, and even 1200-grit or higher. The key is to never skip more than one grit level at a time; otherwise, you’ll be trying to sand out deep 80-grit scratches with 220-grit paper, which is a frustrating and losing battle.

3M Wetordry Sandpaper: The Pro’s Go-To Choice

If you walk into any professional boatyard, you’re going to see stacks of 3M Wetordry paper. There’s a reason it has been the industry standard for decades. Its versatility is its greatest strength; you can use it for aggressive dry sanding on filler or for delicate wet sanding on the final gelcoat finish.

The real magic is in its durability. Cheaper papers often disintegrate when they get wet, turning into a pulpy mess just when you need them most. 3M’s backing is tough and waterproof, and the abrasive particles are bonded exceptionally well, meaning they cut consistently for longer. This reliability saves you time and frustration, especially during the critical final sanding stages where a single rogue piece of grit can ruin your finish.

While it might cost a bit more per sheet than generic hardware store brands, the performance payoff is huge. It clogs less, cuts faster, and lasts longer. For a small boat owner who wants a repair to be indistinguishable from the original hull, using what the pros use is a smart investment.

Norton Black Ice: Best for a Flawless Finish

When your goal is a true mirror shine on your gelcoat or two-part paint, Norton Black Ice is the tool for the final steps. This paper is specifically engineered for finishing work. It’s known for its incredibly consistent and uniform grit, which is essential when you’re working in the 1000 to 2500-grit range.

This is purely a wet-sanding product. Its waterproof latex backing is flexible, allowing it to conform to slight curves without creasing, and it holds up to the prolonged soaking required for color sanding. The goal at this stage is to remove the "orange peel" texture from fresh gelcoat and eliminate any remaining fine scratches from the 600 or 800-grit stage.

Imagine you’ve just sprayed your final gelcoat layer. You’d start with 600 or 800-grit to level the surface, then switch to Norton Black Ice. You’d work your way through 1000, 1500, and maybe even 2000-grit, keeping the surface wet the entire time. The result is a perfectly flat, hazy surface that, once hit with a good compound on a buffer, will explode into a deep, flawless gloss.

Mirka Abranet: Top Pick for Dust-Free Fairing

Traditional sanding creates a massive amount of fine, toxic dust. Fairing a hull repair can leave your workspace—and your lungs—coated in a layer of white powder. Mirka Abranet completely changes the game. It isn’t paper; it’s a mesh abrasive net that allows for near-perfect dust extraction when attached to a vacuum.

The open mesh structure means there’s nowhere for dust to build up and clog the abrasive. The dust is simply pulled straight through the material and into your shop vac. This not only creates a much cleaner and healthier work environment but also makes the abrasive last significantly longer and cut more efficiently. You can see exactly what you’re sanding without constantly stopping to wipe away dust.

The tradeoff is that it’s a system. You need a sander with a hook-and-loop pad and a vacuum hose attachment to get the full benefit. However, for anyone doing more than a tiny spot repair, the investment is well worth it. Fairing a large area on a hull or deck goes from being a miserable, dusty chore to a clean and precise operation.

3M Sanding Sponges: For Contours and Curves

Boat hulls are full of complex curves, tight corners, and non-flat surfaces where a standard sanding block is useless. Trying to sand a curved transom corner or the area around a deck fitting with a hard block will inevitably create flat spots and uneven results. This is where flexible sanding sponges are indispensable.

These sponges feature an abrasive coating on a soft, pliable foam block. They conform perfectly to contours, ensuring even pressure and a consistent scratch pattern across the entire surface. They are perfect for scuffing up old gelcoat in preparation for a new coat, breaking the sharp edge of a repair, or sanding in tight channels and around hardware.

They work equally well for wet or dry sanding. When used wet, the foam holds a good amount of water, helping to lubricate the surface and clear away sanding debris. Keep a few different grits on hand—a medium for general prep and a fine or extra-fine for delicate finish work. They are the secret to professional results on the parts of the boat that aren’t big, flat panels.

Diablo SandNet Discs: Durability for Sanders

For those of us using a random orbital sander for the heavy lifting, Diablo SandNet discs are a fantastic and durable option. Similar in concept to Mirka’s Abranet, these are mesh discs designed for maximum dust extraction and an incredibly long life—often claimed to be 10 times longer than standard paper discs.

Their key advantage is resistance to clogging, especially when sanding gummy materials like old bottom paint or resin-rich fairing compounds. The open-net design allows particles to pass through to the sander’s dust collection system instead of loading up on the disc’s surface. Better yet, you can rinse them with water to clear stubborn clogs and reuse them, making them highly cost-effective for big jobs.

If you’re stripping a large section of hull or fairing an entire deck, the cost savings and reduced downtime from constantly changing clogged discs are significant. They provide a consistent cut for longer, which leads to a more uniform surface finish. They represent a smart balance of performance and accessibility, as they’re widely available in most home improvement stores.

Gator Finishing Paper: Reliable All-Around Use

Sometimes you don’t need a high-tech, specialized abrasive system. You just need a reliable piece of sandpaper that gets the job done without a fuss. Gator Finishing Paper is that dependable workhorse. It’s readily available, affordable, and performs consistently for a wide range of general-purpose tasks.

This is the paper you grab for quick scuff-sanding before applying sealant, knocking the gloss off an area before painting, or for the initial dry-sanding stages of a small filler patch. While it may not have the longevity of a 3M product when wet sanding or the dust-free properties of a mesh abrasive, it’s perfectly capable for the vast majority of DIY boat repairs.

Don’t mistake affordability for poor quality. For dry sanding, it holds up well and provides a consistent scratch pattern. It’s a great choice for stocking your onboard toolkit for emergency repairs. Matching the tool to the task is key, and for many jobs, a simple, effective solution like Gator is all you really need.

The Wet vs. Dry Sanding Technique for Gelcoat

Understanding when to sand wet versus dry is fundamental to a good fiberglass repair. They are two different techniques for two different purposes, and using the wrong one at the wrong time will lead to poor results. It’s not a matter of preference; it’s a matter of process.

Dry sanding is for shaping and speed. This is what you do in the early stages with coarse grits (like 40-180). You use a random orbital sander or a sanding block to quickly shape body filler, grind down high spots, and feather out the edges of a repair. It’s fast and aggressive, but it creates a lot of dust and leaves a relatively rough surface with deep scratches.

Wet sanding is for finishing and finesse. Once your shape is perfect and you’ve applied your topcoat of gelcoat or primer, you switch to wet sanding. By using water with a waterproof sandpaper (like 3M Wetordry or Norton Black Ice), you are lubricating the surface. This prevents the fine paper from clogging with material and, most importantly, it produces a much finer, more uniform scratch pattern. The water floats away the debris, keeping the air clean and allowing you to create a surface that’s ready for polishing to a high gloss. You simply cannot achieve a professional, mirror-like gelcoat finish without wet sanding.

In the end, the sandpaper you choose is as important as the resin or filler. It’s not just about making something smooth; it’s about following a process of refinement. By starting with the right coarse grit for shaping and progressively moving to the right fine-grit paper for finishing, you control the final quality of your work, ensuring your repair is strong, seamless, and a true testament to a job done right.