Understanding Polyester Resins and Gelcoats

A very common question we get asked is should I order waxed or unwaxed gelcoat / resin. We are also asked about different options for getting that hard-tack free finish for the final layer. We get these questions so frequently that we decided to put together a blog post focusing specifically on this topic. We hope you find it helpful.

What is Polyester Resin?

Polyester resin is a versatile, synthetic material widely used in the marine industry, construction, and various DIY projects. It’s known for its excellent mechanical properties, ease of use, and cost-effectiveness. With its ability to harden quickly when combined with a catalyst, polyester resin forms a durable, rigid structure that can be used for a variety of applications.

The Importance of Gelcoat

Gelcoat, a specialized form of colored polyester resin, serves as a protective outer layer for fiberglass structures, especially boats. It enhances the durability, appearance, and waterproofing of these surfaces. Gelcoat can be applied in multiple colors and finishes, making it ideal for both aesthetic and functional purposes. It also provides UV protection with UV inhibitors that helps the gelcoat not discolor and protects other layers from sun rot.

Common Uses

In the marine industry, polyester resins and gelcoats are essential. They’re used in the construction, repair, and maintenance of boats, kayaks, and other watercraft. They are also used in autobody repairs, molds and a variety of other general purpose fiberglass projects. Gelcoats provide a tough, water-resistant finish that can withstand harsh marine and other environments, making it a go-to choice when working with fiberglass.

Waxed vs. Unwaxed Resin and Gelcoat

What is Waxed Resin/ Gelcoat?

Waxed resin contains a paraffin wax additive. When the resin cures, the wax rises to the surface, forming a tack-free layer. This prevents the resin from remaining sticky and allows for easy sanding and finishing for the final layer.

When to Use Waxed Resin/ Gelcoat?

Waxed resin is best for final layers where no additional layers will be added. It's commonly used in repairs and projects where a smooth, tack-free surface is desired immediately after curing. The wax layer prevents inhibition from air, ensuring a complete cure every time. If you have to add a layer over top of the waxed resin/ gelcoat, you will need to sand it, before applying the additional layer.

What is Unwaxed Resin/ Gelcoat?

Unwaxed resin is usually referred to as a laminating resin. Unwaxed means that it does not contain the paraffin wax additive. It remains tacky after curing, which is beneficial for creating strong bonds between layers. This tackiness ensures that subsequent layers of resin or gelcoat adhere properly and do not require sanding in between.

When to Use Unwaxed Resin/ Gelcoat

Unwaxed resin/ gelcoat is best used as a base layer or when additional layers will be added. Its tacky surface allows for better adhesion, making it ideal for laminating and multiple-layer applications. It's the go-to choice for professionals working on complex fiberglass projects.  It is typically the first layer when building a part in a mold.

Creating a Tack-Free Finish Without Waxed Resin

Additives for Tack-Free Finishes

There are several additives available that can be mixed with unwaxed resin or gelcoat to achieve a tack-free finish without the need for a pre-made waxed resin. These additives work by creating a barrier that prevents air from inhibiting the curing process.

Surface Seal

A wax additive that can be added to unwaxed resin or gelcoat is called surface seal. Four ounces per gallon will be needed. This is mixed in before the catalyst and will create a hard tack free finish.

Surface Sealants

Surface sealants, such as PVA (Polyvinyl Alcohol), can be sprayed onto the surface of the resin or gelcoat. They form a film that blocks air contact, allowing the resin to cure fully and tack-free. Once cured, the sealant can be washed off, leaving a smooth surface ready for further processing.

Conclusion

Choosing the right resin and gelcoat is vital for achieving the best results in your marine and fiberglass projects. Waxed resin provides a convenient, tack-free finish ideal for final layers, while unwaxed gelcoat offers strong adhesion necessary for multi-layer applications. By understanding the benefits and applications of both materials, you can make informed decisions that enhance the quality, durability, and aesthetics of your work.

For those looking to elevate their fiberglass projects, consider exploring advanced materials and techniques. Whether you're a marine industry professional, boat owner, or DIY enthusiast, the right choice of resin and gelcoat can make all the difference.

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