The quick answer is no. You usually can’t fiberglass over existing paint for several reasons. If you’re restoring an old boat, for example, and the hull is painted, you cannot apply fiberglass over the paint. Or at least not without much prep work.
Whether you’re looking to get rid of an old, peeling paint job or cover up a mistake, you may be tempted to use fiberglass over paint. On the surface, this sounds like a great idea — the fiberglass is thick and should fill in all those pesky cracks. But on the second inspection, it becomes apparent that you should never do this.
Fiberlight is made from glass fibers and epoxy resin; paint is made from acrylics and polyurethane resins. Because the two are chemically incompatible, whatever you apply over the paint will eventually fail.
This is why many people who use fiberglass over paint have to sand off the coating to get down to the primer. Many amateur carpenters also use fiberglass over paint to fill in nail holes. The problem with this approach is that nails are wood, and wood has different curing times than fiberglass. The nail holes will be cured before the fiberglass, and you’ll have a mess.
When applying fiberglass, you want to work with as little paint as possible. Paint adds resistance to the resin and often prevents a good cure. Plus, it’s hard to get sandpaper to adhere to painted surfaces, which means you’ll never be able to get a smooth finish once you’ve cured the glass mat.
What is fiberglass?
Fiberglass is a material that can be used to make all sorts of materials, including boats, cars, and homes. The word “fiberglass” comes from the fact that it’s made from fibrous glass fibers.
Fiberglass is an extremely strong material that’s much more durable than wood or metal, offering strength and insulation. It’s also lighter than wood or metal and won’t burn or rot like wood or metal.
One of the most common uses for fiberglass is in boat building. Fiberglass boats are extremely durable and can withstand the hardest of water conditions.
How do you fiberglass?
Fiberglassing is all about layering material so that you have a thick, strong surface on which to work. The process begins with applying a layer of resin over a surface and then laying a mat or cloth over that to act as an “interlocking” agent between the resin and the surface beneath it.
This layered material is then heated and applied to the interior of the surface you are fiberglassing. The resin usually takes an hour to two hours to fully cure, but you mustn’t leave it on longer than this.
For large surfaces such as boats, houses, and cars, a suspended ceiling is usually used as the matting layer. The ceiling is suspended, so it’s very easy to access the fiberglassed ceiling area. The matting layer adheres to the surface with resin for small areas such as a walkway or a large table.
If you have to repair painted surfaces, what should you do?
First, sand off as much paint as you can. Then, if exposed metal surfaces (i.e., the hull has been galvanized), prime those with epoxy primer to ensure a good bond between the paint and the glass mat.
Can you glaze fiberglass?
You can glaze fiberglass, but it’s not advisable to do so unless you have to. Glazing means coating the mat with a second layer of woven fiberglass fibers bonded with resin. After the resin has cured, the woven material becomes extremely strong.
The main advantage of glazing is that it adds strength to the overall material. The downside is that you can’t strip off the glaze once the resin has been applied and cured. If there are areas where it hasn’t been applied thoroughly or evenly, you won’t be able to remove the glaze from those areas.
The other issue with glazing is that it’s difficult to do without a spray gun. Doing so by hand is extremely tedious and time-consuming. Spray-gun installation is much more feasible but requires a professional-grade gun and materials.
Can you recover fiberglass?
Yes, you can recover fiberglass if the surface is bare or has a primer or glaze coating. However, the resin has a limited lifetime. It loses its strength after about 10 to 15 years, even if you don’t use it outdoors.
The best way to extend the life of your resin is to cover it with a cloth material (usually woven glass fabric). By doing so, the resin won’t come into touch with the air, which reduces deterioration.
Step-by-step guide on how to apply fiberglass
The process of applying fiberglass is pretty easy. You will need the following materials to have a smooth sail when applying fiberglass:
- Fiberglass cloth
- Polyester resin and hardener
- Duplication mold
- Metal Bowl
- Wax paper
- Wooden paint stick
Step one: Make the mold.
If you are creating fiberglass for a project, such as a straightforward box, bowl, or other shapes, you should put together a “mold” or “form” to ensure that your fiberglass begins in a liquid state and sticks to the proper shape. Consider tape off the repair site and applying the resinous fiberglass coating directly to the repair site if you need to do a fiberglass repair, as you might on a boat or automobile.
Step two: Prepare the Fiberglass Cloth for Use.
To cover the mold, cut any fiberglass fabric into the proper proportions. Make sure there is enough room for overlap to maintain the product sturdy and long-lasting.
Step three: Combine the resin and fiberglass.
In the metal bowl, carefully pour the right amount of resin and hardener. Mix the solution completely, being sure to get the edges and bottom of the container using the paint stick.
Step four: Apply the resin mixture after placing the fiberglass on the mold.
Spread the resin evenly over the fiberglass cloth, covering any corners and weak areas with the same amount of resin as you would over level, accessible areas. For instance, your fiberglass may develop flaws in those corners if you don’t obtain adequate cover.
Step five: Continue applying until the desired thickness is reached.
For the result to be as sturdy as possible, take the time to ensure each layer is positioned in a distinct direction. Sand in between steps to remove any ridges where the mat or cloth may poke through the resin. Finish the last layer and let the application dry completely.
Step six: Apply a smooth resin coat to the entire surface
This process will even take it out. Then, if desired, paint with polyurethane or alkyd enamel.
Step seven: Take the mold out of the finished fiberglass shape.
You should be able to separate the form from the shape or peel the shape off of the form if your form or mold has been wrapped in wax paper or paraffin wax. The wax won’t allow the fiberglass to adhere.
This is a short step-by-step guide on how to apply fiberglass. However, you can use many other methods for applying fiberglass to a surface. You’ll have to put in the time and effort to get your fiberglass project right.
Can you apply fiberglass over wood?
You can glass over wood if it’s in good condition. The plywood surfaces on most boats are usually in good shape. If they weren’t, the boat would have sunk by the time the owner gave up on it! That said, there might be areas where the plywood is warped. If you try to lay down a glass mat on a warped surface, the mat will stretch when it cures. Once the resin sets, you’ll have a big hump in your hull.
Before you lay down any mat, you want to have a smooth surface for the resin to cling to. You need to sand the surface down to about 100 grit to achieve that. Once you’ve covered the wood with a mat, you won’t be able to sand it anymore, so you must get it right the first time.
Recommendations on using fiberglass
Once your paint job, priming, glazing (if necessary), and resin application are all completed, your boat hull will be extremely durable and resistant to corrosion, abrasion, and shock. Because of these qualities, fiberglass is very popular for boat construction. That said, it does have a few drawbacks, most of which can be avoided by hiring a professional.
If you’re committed to using fiberglass, you’ll be happy to know that it’s the best material for boats and yachts. If you want the ultimate durability, especially for boats that see a lot of action, fiberglass is what you should use. Fiberglass is also great for car repair and home improvement since it’s easy to repair. When it’s not quite right, you can call the pros, and they’ll fix it for you right then and there.