Fairing the boat hull below the waterline with epoxy putty

In this article, we explain how to fair your boat hull below the waterline after a major repair of holes and cracks. This is done easily by priming the boat. The guide is also applicable if you are not satisfied with the shape or smoothness of the surface. You can use this guide to prime polyester, epoxy, wood, steel or aluminum boats. Before you start, it is important to clean, strip and sand the boat first. Before doing so, please read the articles we have written on this subject.

You can fair your boat hull with epoxy putty by following these five steps:

• Step 1. Marking uneven areas.
• Step 2. Dry sanding and degreasing the substrate.
• Step 3. Applying a first coat of epoxy filler.
• Step 4. Sanding, checking and refilling.
• Step 5. Follow up work.

Supplies to putty your boat

To prime a boat you need the following materials and tools. You can order all supplies easily online at Polyestershoppen.com. If you order before 5 p.m. Dutch time and all products are in stock, we will send your order the same day so you can get started as soon as possible. 

Raw materials:

• Variopox Epoxy Putty

Tools:

• Sanding paper (40, 80, 120, 180, 240 grit)
• Dust mask
• Sander + sanding discs
• Kitchen scale
• Nitrile gloves
• Putty board
• Putty knives or putty rubbers

The right amounts depend on the overall size of your boat hull. Not sure about the quantity needed? Then feel free to contact our specialists for tailor-made advice. 

Boat fairing: a step-by-step guide

In the video below, our product specialist Arne shows how to fair the underwater hull with epoxy filler. Below the video, you can read a more detailed step-by-step plan in which we explain in 5 steps how to plaster a boat.

Step 1. Mark uneven areas

Use an old tube light (without fixture). Grasp it in the middle (with 2 fingers and thumb) and slide it slowly across the hull. At uneven spots you can feel the tube light flip over. Mark all these uneven spots (e.g. by scratching with a pencil). 

Step 2. Dry sand and degrease the substrate

Dry sand the marked areas with a sander (P60 grit). Then degrease the substrate with Natusafe biological degreaser. This will improve the adhesion between the substrate and the putty.

Step 3. Apply the first coat of epoxy filler

Now that the substrate has been properly prepared, we continue with applying the first layer of epoxy filler. We use Variopox Epoxy Filler for this purpose. This two-component epoxy putty is easy to sand and perfectly suitable if water resistance is important. Therefore, this filler is appropriate as a boat filler and can be applied both above and below the waterline.

Weigh both components according to the mixing ratio 2:1 on a digital scale and mix the components to an even colour. Then apply the filler with a putty knife, flexible spatula or Japanese spatula. Always use nitrile gloves when working with epoxy.

Step 4. Sand, check and re-fill

Make a sanding board from a piece of plywood and 2 good handles. Glue or pin 60 grit (dry) sandpaper to it and sand the hull using diagonal strokes (left and right). You will soon see where filler is still needed. Apply more filler where necessary to fair the hull.

Use the epoxy filler to fill in any air bubbles, dimples and stripes that occurred during stripping. Finally, sand the surface with a fine grit so that the hull is nice and flat and smooth.

Step 5. Follow-up 

Now the hull and the underwater hull are nice and smooth, please continue with the next steps. We wrote the following articles about (applying) antifouling:

• What is antifouling?
• Applying hard antifouling.
• Applying self-sharpening antifouling.

Want to buy boat filler?

All the products you need for this job can be ordered at Polyestershoppen.com. We supply boat filler at lightning speed from stock so you can quickly get started on your project.

Do you have questions about fillers or boat maintenance? Please feel free to contact our product experts. We are happy to give you honest advice to make your project a success. you can email your questions to support@polyestershoppen.com or send us a WhatsApp message!

Frequently asked questions about boat fairing

Our experts receive many questions about fairing and filling boat hulls. Here are the answers to the 3 most asked questions:

Is it necessary to sand before applying filler?
Before applying filler, it is important to sand uneven areas. After sanding, use a degreaser for ideal adhesion between the substrate and the filler. It is also important to continue to sand between layers of filler.

How do I remove any imperfections in the filler after curing?
Not quite satisfied with the result? Then sand diagonal lines with 60-grit paper. This way you can see where more filler is needed. Apply more filler where necessary to fair the hull. 

How do I determine the correct thickness of the filler?
The correct amounts depend on the overall size of your boat hull and its current condition. Are you unsure about the quantity needed? Please feel free to contact our specialists for personal advice.