Fairing Compounds and Repair Putties FAQs
What is fairing compound and what is it used for?
A fairing compound is typically composed of epoxy resin and low-density fillers, and is used to fill and shape low areas on surfaces like fiberglass, wood, steel, and aluminum. Fairing a surface creates a fair appearance that looks and feels smooth while blending with the surrounding area.
Fairing compound is often used prior to fiberglassing to ensure an even surface, and because it sands easier than fiberglass. Surface fillers, fairing compounds, and repair compounds are used to fill large cavities, cover large areas, and provide a smooth surface for subsequent processes. Fiberglass hulls frequently require minor touch up repairs to address scratches and other surface defects like small hull dents, dings, and gouges prior to priming and painting the surfaces.
Common repairs below the waterline, like reshaping and fairing a boats rudder and keel, or filling holes after grinding down gelcoat blisters, are very common fairing applications.
What types of resin are used in fairing compounds?
Each type of fairing compound contains an epoxy, polyester, or vinyl ester resin. Each resin is designed for specific boat construction and repair tasks.
In addition to the resin, each compound contains a substance designed to activate the resin and ensure that it cures to a hard plastic material. For example, epoxy resin is paired with a hardener. Polyester and vinyl ester resins depend on a catalyst to complete the curing process. Always use the manufacturer-specified amounts of hardener or catalyst, or the resin will not cure as intended.
All fairing compounds also feature a filler component. For example, an epoxy resin-based fairing compound contains fumed silica. Polyester- and vinyl ester-based compounds generally contain fiberglass strands or chopped fiber, both intended to thicken the putty so it will not sag or run.
Every fairing compound also contains an additive, such as glass microballoons, that makes sanding easier after it cures.
Can you make your own fairing filler?
Yes! Making your own fairing filler lets you customize the fillers properties. You will need an appropriate epoxy resin system, thickener like colloidal silica, and microballoons to make it easier to sand. Tip: always mix resin and hardener before adding additives or filler to the mixture. In general, a pre-mixed, pre-packaged product is a faster, easier, and less expensive method.
What is the difference between epoxy, polyester, and vinyl ester resins?
Epoxy fairing compound
Compatible with fiberglass, wood, and metal boat hulls when primer and/or paint are applied over the compound. It can be used below or above the waterline. Compared to the other resin types, epoxy resin-based compounds provide improved adhesion, superior strength, and more resistance to water penetration (also called osmosis). This is the best fairing compound for wooden boats.
Polyester resin fairing compound
Ideal for above-the-waterline fairing on fiberglass boats. These compounds also excel at filling dents, dings, and gouges. After applying the compound, the user must finish the surface with gelcoat or prime using an epoxy primer, and then paint with a topcoat.
Vinylester resin compound
Used for fairing above or below the waterline on fiberglass vessels. Vinylester resin also fills dings, dents, and gouges. After the fillers are applied, the surface must be finished with gelcoat or primed and painted. Compared to polyester resin, vinylesters show less cracking and shrinkage.
What is the difference between Bondo and fairing compound?
Bondo type fillers cannot be compared to marine fairing compounds, as Bondo is not waterproof, and lacks the strength of marine epoxy and vinyl yester repair products. It should not be used below the waterline.
What tools or equipment do you need to fair?
To prep the surface, you will use tools like an angle grinder, and a rotary tool like a Dremel. You will need a trowel or spreader, notched spreader, a scraper, sanding block, flexible long board sander (the long board makes sanding large areas a lot easier) to sand afterwards, and sandpaper.
Generally, 60 to 180 grit sandpaper is used for surface prep, and 220 to 320 to sand cured compound smooth. Be sure to mask off adjacent areas with a good tape that allows clean removal. Use clean rags and denatured alcohol to wipe down surfaces. Always protect yourself by wearing a suitable NIOSH approved respirator, eye protection, protective clothing, and gloves.
How do you use fairing compound?
Begin with a clean surface to improve adhesion, and will also ensure that the compound remains in place after it is cured.
- Clean the hull surface with acetone, denatured alcohol, or other solvent. Use the two-rag wipe on/wipe off technique: apply the solvent with one rag, and remove it with the clean rag. Change rags often.
- Grind any surface cracks, crazing cracks, or gelcoat blisters. Grinding and beveling the cracks and blisters will improve the fairing compound bond.
- Mask areas that are off-limits for sanding.
- Sand the surface to improve fairing compound adhesion. Use 60 to 180 grit sandpaper, and sand down until you reach solid material.
- Remove the sanding residue with a vacuum cleaner.
- Wipe the area again with denatured alcohol or acetone.
- Mix the fairing compound according to the manufacturers instructions.
- Apply the fairing compound with a plastic spreader. Smooth the compound so it is close to the desired shape, and blend it into the surrounding surface.
- Allow the fairing compound to cure completely before sanding.
Sand the surface until it is smooth. This will help the primer to adhere better.
Remove sanding residue with a shop vac. Wipe down the surface with a lint-free clean cloth and denatured alcohol or acetone.
Can you apply fairing compound over gelcoat?
Yes, you can apply fairing compound over gelcoat. Sand the hull and wipe it down before application. Note that polyester fairing compound is only designed for above-the-waterline use. Epoxy fairing compound is suited for above-the-waterline and below-the-waterline applications.
Can you apply gelcoat over the fairing compound?
Gelcoat should not be applied directly over epoxy fairing compounds like TotalFair. Doing so will result in the gelcoat failing to cure properly. In order to gelcoat over fairing compound, you need to first apply a 2-part epoxy primer.