How to Sand the Bottom of a New Boat Before Painting

Sanding the bottom of a new boat is crucial for efficiency and performance. Whether you operate a fast powerboat (which has a planing hull) or a sailboat (which has a displacement hull), having a smooth bottom promotes the "laminar flow" that makes the underwater foils (keel, skeg) work and minimizes the drag that reduces speed and saps power and fuel economy. New boat bottoms can be quite rough and have pits, hollows and toolmarks that must be faired, or smoothed out, before the final bottom paint is applied.

Things You'll Need

• Fairing (smoothing) compound
• Circular (orbital) sander
• Circular sandpaper, 60 to 80 grit
• Sticky-back sandpaper, 100 to 120 grit
• Putty knives (2-inch, 4-inch and 6-inch)
• Sanding block
• Fairing board (strip of 4 x 24 x 3/4-inch plywood)
• Masking tape
• Black spray paint

Instructions

Rough-in

1. Mask the boot top (waterline stripe) with tape. Make sure the tape is well stuck on the bottom edge, to protect the topsides (the part of the hull that is above the waterline) and to prevent bottom paint bleed-under during the painting procedure.

2. Sand the entire bottom with a circular sander to remove any bumps or offending material. Start at one end of the boat and move down one side of the bottom, following a circular pattern that overlaps at the edges and gives a uniform swirly appearance to the bottom. Repeat the process on the other side. Be careful to move at a consistent speed, so as not to remove too much material and leave divots that will cause much added labor later.

3. Mix up a small batch of fairing compound according to the manufacturer's instructions. Trowel on the fairing compound with the biggest putty knife that will work with the curvature of the hull. Be careful to avoid gouging the surface with a knife corner, as it will create more work later. Small batches will help to prevent compound waste if you run into a delay while troweling it on or if ambient temperatures are high.

4. Sand the bottom again with the circular sander and pattern, removing all surface compound. This will leave little 'dots' of compound in any pits or flaws in the bottom and will show where to focus the next phase of fairing.

Fine Tuning

1. Build a fairing board by covering one side of the plywood strip with sticky-back sandpaper. Ensure that no edges of the paper overlap; that will create "hot spots" during fairing.

2. Apply a very light mist of black spray paint to the bottom of the boat---just a dusting that will leave a salt-and-pepper appearance.

3. Sand the bottom with the fairing board. Begin at one end of the boat at the boot top. Move the board down from the boot top to the keel and back up, while slowly creeping toward the other end of the boat. Keep the fairing board horizontal to the boot top, and slide the board lengthwise along the bottom, leaving only fore and aft scratches. Make a light pass over the bottom to remove only the high spots in the indicator paint. The paint that remains will indicate any low areas in the surface.

4. Mix more fairing compound in small batches and fill any remaining depressions, using the widest knife possible. Allow the compound to dry well and repeat the fairing-board process. Mist again with spray paint and repeat the fairing pattern.

5. Repeat this process until all the indicator paint can be removed by the fairing board. The bottom will now be perfectly filled and in plane with itself. The bottom is ready for paint and will be visibly smooth, with no imperfections or drag-inducing shapes.

Tips & Warnings

• Wear eye protection and respiratory protection during sanding. Some very dangerous and poisonous materials are used, especially in old bottom paint, that you will do well to avoid inhaling. Be patient. The fairing-board process can seem dreadfully slow and labor intensive, but it is the only way to achieve a good, "fast" bottom.

References

• Bottom Painting Tips
• New Fiberglass Bottom