Wet Sanding An Aluminum Boat: The Ultimate Guide

Wet sanding is a meticulous process that can restore the shine of your boat's gel coat finish by removing oxidation. It is a cost-effective and DIY-friendly method to enhance the appearance of your boat. To wet sand your aluminium boat, you will need a random orbital polisher, an interface pad, sanding discs of varying grits, a water source, a guide coat, and optionally, a gloss meter. The process involves applying a guide coat to identify high and low spots, using the polisher with an interface pad and sanding discs, and alternating sanding directions. It is important to keep the surface wet and rinse frequently to prevent residue build-up. While wet sanding is accessible, it requires patience, precision, and an understanding of the process to achieve the desired results.

Wet sanding equipment

Wet sanding is a process that can be done by hand or by machine. It is used to remove paint, smooth out corrosion spots, and prepare a boat for repainting. Here is a list of equipment that can be used for wet sanding an aluminium boat:

Hand Sanding:

• Sandpaper: Sandpaper with a grit of 400, 600, 80, 120, 150, 180, 320, or 1000 can be used, depending on the desired finish and the condition of the boat. Finer grits will give a smoother finish but will require more stages of polishing.
• Respirator: It is important to wear a respirator to protect yourself from dust and fumes when sanding.
• Gloves: Wear gloves to protect your hands from the sandpaper and any chemicals used.
• Brushes: Wire brushes or Scotch-Brite pads can be used to remove corrosion and old paint.

Machine Sanding:

• Orbital Sander: A 6-inch, 8mm orbital polisher/sander with an interface pad is recommended for machine sanding. This will help to reduce the possibility of pigtails (small, tight curls) in the sanded surface.
• Sanding Discs: Sanding discs or pads with a grit of 320, 600, or 1000 can be used with the orbital sander. The Mirka Abralon mesh abrasive disc is recommended as it stays clean and does not load up.
• Polishing Compounds: After sanding, polishing compounds can be used to remove fine scratches left by the sanding process. Restructure Heavy Cut Compound and Elevate Medium Cut Compound are recommended for this purpose.
• Buffers: A rotary buffer can be used with a wool pad to apply the polishing compound.

Sanding techniques

Wet sanding is a meticulous process that can restore the shine of your boat's gel coat finish by removing oxidation. It is a cost-effective and DIY-friendly method to enhance the appearance of your boat. Here are some essential sanding techniques to guide you through the process:

Equipment and Materials:

• Invest in a good random orbital polisher, preferably a 6-inch polisher, for efficient and even sanding. The Griots Garage G9 is a recommended option.
• Use an interface pad, such as the 6-inch Mirka interface pad, to provide cushioning and prevent excess scratches.
• Gather sanding discs in various grits, such as 500, 1000, and 2000 grit Mirka Abralon pads, depending on your boat's condition.
• Ensure a constant water flow during the process, as it is crucial for wet sanding.
• Consider using a guide coat, a contrasting chalk application, to help identify high and low spots on the gel coat.
• A gloss meter is optional but can provide valuable insights into your gel coat's condition and guide your grit selection.

Understanding the Guide Coat:

• Apply the guide coat evenly across the surface you intend to sand.
• Start sanding with the lowest grit and use the random orbital polisher with an interface pad.
• Focus on the "peaks," the areas where the guide coat wears off first, and sand them down to the "valleys" for an even surface.
• Continue sanding until the guide coat is uniformly removed, indicating that the surface is ready for the next grit.
• Repeat the process with each grit, using the guide coat to minimise scratches.

Interpreting Gloss Meter Readings:

• While not mandatory, a gloss meter can help you assess your gel coat's condition.
• Readings of 20-40: Start with 1000 and 2000 grit Mirka Abralon sanding discs.
• Readings of 10-20: Begin with 500 grit, followed by 1000 and 2000 grit.
• Readings of 0-10: Start with 800-1000 grit and then progress to 500, 1000, and 2000 grit.

Wet Sanding Process:

• Maintain a constant flow of water during sanding to keep the discs effective and prevent clogging.
• Use a random orbital polisher for even sanding and to reduce the risk of over-sanding.
• Alternate between vertical and horizontal passes to ensure thorough sanding and an even finish.
• Rinse both the surface and the sanding pad frequently to remove residue.
• For areas that the machine can't reach, such as under cleats or around railings, hand-sand using the same grits.

Tips and Precautions:

• Don't rush the process. Maintain a consistent and controlled pace to avoid over-sanding.
• Always use an interface pad to prevent unnecessary scratches.
• Keep the pad flat against the surface and avoid excessive tilting.
• Turn on the polisher when it's in contact with the surface, and let it come to a complete stop before lifting it off.
• Replace sanding discs frequently to maintain optimal performance and avoid grit breakdown.
• Gel coat is thicker than you think, so don't worry too much about sanding through. Modern abrasives are generally safe.

Sanding scratches

Sanding an aluminium boat is a challenging task, and it is important to be cautious to avoid damaging the hull. Before attempting to sand out scratches, it is recommended to try using a chemical stripper to remove paint or corrosion. If this is not effective, then sanding can be considered.

For light scratches on an anodized aluminium surface, it is necessary to cut through the top layer and remove the anodizing in the scratched area. This can be done using an abrasive polish, such as Nuvite G6, or with power tools like a Scotch-Brite wheel followed by a buffing wheel. However, removing the anodizing layer will leave the aluminium unprotected and susceptible to corrosion. To mitigate this, the area can be clear-coated, but this will have a limited lifespan due to UV exposure.

For deeper scratches or gouges, particularly those below the waterline, it is crucial to make a robust repair to ensure watertight integrity. One method is to use an epoxy adhesive like PC-11 or Marine Tex, applied externally and/or internally, to fill and seal the damaged area. Proper surface preparation is critical for a strong bond, including wire brushing, cleaning with vinegar and water, and thorough drying. Painting the repaired area will enhance UV protection. Another option for repairing deep scratches is welding, but this requires access to the opposite side of the puncture and skilled technique to avoid warping or further damage.

When sanding aluminium, it is generally recommended to use a machine sander, such as a 6-inch 8mm orbital polisher/sander with an interface pad, to achieve the best results. For hand-sanding, use a grit of around 400, and always prime the aluminium before painting.

Sanding and corrosion

Wet sanding is a great way to restore the finish of your aluminium boat. However, it's important to be aware of the risks of corrosion and how to prevent it.

Aluminium boats are corrosion-resistant but not corrosion-proof, especially if low-grade aluminium is used. Corrosion can occur when a boat is moored, and saltwater poses a more extreme risk of galvanic corrosion, electrolysis, and crevice corrosion. Galvanic corrosion occurs when two dissimilar metals come into contact with each other in an electrolyte solution, such as saltwater. Electrolysis is caused by potential current (AC or DC) between two different objects, often due to stray electrical current in the water. Crevice corrosion happens when moisture is trapped in a crack or crevice, leading to a breakdown of the passive film and subsequent corrosion.

To prevent corrosion, it is recommended to isolate dissimilar metals to reduce the potential for galvanic corrosion. This can be done by using plastic washers and sleeves to separate stainless bolts and fittings from the aluminium hull. Additionally, sacrificial zinc anodes can be attached to the aluminium parts, providing extra electrodes for the metal to shed, thus corroding the zinc before the aluminium. It is also important to only use paint and coatings designed for aluminium, repair any cracks or scratches exposing bare aluminium, and always rinse the boat with freshwater after use in saltwater.

When sanding an aluminium boat, it is crucial to be mindful of corrosion and take the necessary precautions to prevent it. This includes properly isolating dissimilar metals, using sacrificial anodes, and maintaining the boat's finish with freshwater rinses and appropriate coatings.

Sanding and painting

Wet sanding is a meticulous process that can restore the shine to your aluminium boat by removing oxidation and revealing its original vibrant colour. It is a cost-effective and DIY-friendly method to improve the gel coat adhesion and protect your boat.

Preparation:

Firstly, assess the condition of your boat. If there is existing paint, you may need to strip it off using a chemical stripper or aircraft paint stripper. Ensure you are wearing protective gear, including a mask, to avoid inhaling any particles. Once the paint is removed, clean the surface thoroughly.

Sanding:

The next step is to sand the aluminium surface. You can use a random orbital polisher or a sanding board for this process. If using a polisher, a 6-inch polisher with an interface pad is recommended to prevent excess scratches. For the sanding discs, you may need various grits, such as 500, 1000, and 2000. It is important to keep the sanding discs wet during the process to prevent them from becoming ineffective.

Start with the lowest grit, such as 500, and use vertical and horizontal passes to ensure thorough sanding. Rinse both the boat and the sanding pad frequently to remove residue and prevent clogging. Continue this process, increasing the grit with each pass. Hand-sand difficult-to-reach spots, such as under cleats or around railings, using the same grits.

Painting:

Once you have finished sanding, it is important to clean the surface thoroughly to remove any dust or residue. You can use a vacuum and paper towels with naphtha for this step. After the surface is clean and dry, it is time to prime the aluminium. Use a primer suitable for aluminium, such as zinc chromate, and apply it according to the manufacturer's instructions.

Finally, you can begin painting your boat. Choose a paint that is specifically designed for aluminium boats and follow the instructions for application and drying time. It is important to work in a well-ventilated area and wear protective gear during the painting process.

By following these steps, you can achieve a smooth and professional finish on your aluminium boat, protecting it from the elements and giving it a like-new appearance.

Frequently asked questions