How to Waterproof a Wooden Hot Tub

Hot tubs are extremely popular for enjoying a relaxing soak in your own garden. Most hot tubs are made of wood, a material that not only provides excellent insulation but also gives a warm, rustic charm to any outdoor space. Thanks to wood's natural insulating properties, the water stays warm longer, meaning you need less energy or firewood to keep the hot tub at the desired temperature.

A wooden hot tub remains watertight as long as it stays continuously filled with water. However, the water needs to be refreshed periodically. When emptied, the wood dries out, causing it to shrink and potentially develop leaks. Many customers ask us how they can restore their leaking hot tub to a fully watertight state. In this guide, we walk you step-by-step through coating the inside of your hot tub with vinylester resin or epoxy resin to make it completely leak-proof once again. This method is straightforward for any handy DIY enthusiast to carry out.

Vinylester resin or epoxy resin?

In this article, we cover two methods for waterproofing your hot tub: using vinylester resin or epoxy resin. The table below outlines the key properties of both materials.

Our recommendation? Use vinylester resin!

When waterproofing a hot tub, we strongly recommend using vinylester resin. Vinylester resin is ideal for hot tubs because of its high resistance to warm water and harsh pool chemicals, including acids and chlorine. Furthermore, vinylester resin is significantly more cost-effective than epoxy. Not sure which option suits your situation best? Contact our specialists — we are happy to provide personalized advice!

Method 1. Coating a hot tub with vinylester resin (colored finish)

Follow the steps below to line a hot tub using vinylester resin and fiberglass mat. All required supplies are available at Polyestershoppen.nl.

Requirements

• RESION polyester bonding primer (coverage: approx. 200 g/m²)
• Fiberglass mat 450 g/m² (2x total hot tub surface area)
• Vinylester resin + hardener (coverage: approx. 1 kg/m² per layer)
• Colored gelcoat + hardener (coverage: approx. 400 g/m²)
• Colored topcoat + hardener (coverage: approx. 400 g/m²)
• Bubble buster roller
• Brushes
• Paint roller + roller frame
• Acetone
• Sandpaper
• Cleaning agents
• General consumables

Step 1. Preparation

Thoroughly clean the hot tub. Scrub the wood well and sand the surface so the resin can adhere properly. Plan the work ahead and lay out all materials before starting.

Begin with the walls and then cover the floor, extending the floor matting 10 cm up the walls for an overlapping seal.

Step 2. Applying RESION bonding primer

Once the hot tub is clean and dry, apply the bonding primer to ensure a reliable bond between the wood and vinylester resin. RESION polyester bonding primer is a user-friendly, single-component product that requires no hardener mixing. Apply it directly to the wood using a flat brush or roller. Depending on ambient conditions, the primer cures sufficiently within 30 minutes to 2 hours for you to proceed with the vinylester resin.

Step 3. Laminating with fiberglass mat and vinylester resin

With the primer applied and cured, begin laminating with vinylester resin and fiberglass mat.

• Mix vinylester resin with approx. 2% MEKP hardener. Weigh accurately and mix thoroughly.
• Pre-wet the substrate with vinylester resin and lay the fiberglass mat into it.
• Saturate the mat using a roller. The mat will soften and turn transparent. Use its flexibility to drape it smoothly around corners and complex shapes. Apply more resin to dry white spots. Avoid excessive resin buildup — the texture of the mat should remain subtle but visible.
• De-aerate after each layer using a bubble buster roller. The laminate will darken slightly as air is removed, making it easy to see where you have already rolled.

Once the first layer is laminated, you can immediately apply the second layer wet-on-wet. De-aerate the second layer thoroughly before allowing the structure to cure for at least 2 hours.

Step 4. Applying gelcoat

Wipe the surface clean with acetone and lightly sand with 60-grit sandpaper to remove raised fibers. After sanding and degreasing, apply a layer of polyester gelcoat (coverage: approx. 400 g/m²). Add approx. 2% MEKP hardener to the base, mix thoroughly, and apply evenly using a roller.

Gelcoat cures with a tacky surface, providing superior chemical bonding with the subsequent topcoat layer.

Step 5. Applying polyester topcoat

In the final step, finish the hot tub with polyester topcoat. Because the underlying gelcoat remains tacky, the topcoat can be applied directly without sanding or degreasing. Combining gelcoat and topcoat creates a thick, resilient barrier that protects the tub against water, UV, and wear.

RESION ISO-NPG Topcoat is easy to apply. Add approx. 2% hardener to the base, mix well, and apply a coat using a roller.

Allow the topcoat to cure for approximately two days. Once cured, your hot tub is ready to be refilled with water!

Method 2. Coating a hot tub with epoxy (clear finish)

Follow the steps below to line a hot tub using epoxy and woven fiberglass fabric. All supplies are available at Polyestershoppen.nl.

Requirements

• RESION UV Clear Coat epoxy — used as primer, laminating resin, and final clear coat (coverage: approx. 1.2 kg/m² total)
• Woven fiberglass fabric 200 g/m² (2x total hot tub surface area)
• Bubble buster roller
• Brushes
• Velour roller + roller frame
• Cleaning supplies
• General consumables

Step 1. Preparation

Clean the hot tub thoroughly. Scrub and sand the wood to create an optimal surface for epoxy adhesion. Organize your work area and prepare materials in advance.

Start on the walls first, then coat the floor, extending the floor fabric 10 cm up the walls for an effective seal.

Step 2. Impregnating the hot tub

After cleaning and sanding, prime/impregnate the wooden interior using epoxy resin.

• Thoroughly mix the clear epoxy resin and hardener at the correct ratio (100:50 by weight), making sure to scrape the sides and bottom of the mixing cup!
• Apply the epoxy to the interior wood surface using a velour roller and allow it to cure.
• Once cured, the wood grain may feel rough. Lightly sand until smooth again before laminating.

Now you are ready to proceed with fiberglass laminating.

Step 3. Laminating with fiberglass fabric

With the wood sealed, begin laminating with epoxy and woven fiberglass fabric.

• Mix epoxy resin and hardener accurately at the specified ratio.
• Pre-wet the substrate with epoxy resin and lay the woven fiberglass fabric into the wet resin.
• Saturate the fabric using a roller until the weave turns transparent. Apply additional resin to white dry spots, avoiding puddles.
• De-aerate thoroughly after each layer using a bubble buster roller. The laminate will darken slightly as air bubbles escape.

• Immediately apply a second layer of fabric wet-on-wet following the same process.

Step 4. Applying the final topcoat

Once the laminate has fully cured, sand lightly with 60-grit sandpaper to remove any protruding fiber tips. Then apply a final layer of clear epoxy coating.

• Mix resin and hardener, then roll out a thin, even coat.
• Apply a second coat roughly 12 hours later.

Allow this clear epoxy finish to cure for 7 full days before filling the hot tub with water!

Need more information about waterproofing a hot tub?

In this article, you learned two reliable methods for making your hot tub completely leak-proof. Do you have additional questions about products, application techniques, or project planning? Our technical specialists are happy to help! Contact us here for expert advice or a free quotation.