How to Sand Fiberglass? [A Complete Guide]

Fiberglass is a plastic shape that is encapsulated with small acrylic glass fibers. For architecture, it’s all right. They are small, compressive, tensile, and easy to shape. Since then, it is famous for being a material for boat ships, car bodies, and even residential buildings.

Its specific characteristics make sanding a little challenging, and it requires a great deal of preparatory work and patience to learn how to sand fiberglass.

Sanding it for extra luminosity or elegance makes it look and sound amazing. Sanding fiberglass can be a precaution for your fiberglass accessories. Don’t worry. By reading this content, you would know how to sand fiberglass.

Table of Contents

  • Protective Equipment
  • Tools and Materials You Will Need
  • Sanding Fiberglass [7 Easy Steps]
  • How to Wet Sand Fiberglass?
  • Do You Have to Sand Between Layers of Fiberglass?
  • What Kind of Sandpaper Do You Use on Fiberglass?
  • Cost to Sand and Paint Fiberglass

Protective Equipment

  • Hand Gloves
  • Eye Goggles
  • Coverall
  • Respirator

Tools and Materials You Will Need

  • Abrasive sponge
  • Wax and grease remover
  • The vacuum
  • A gallon
  • Sanding board
  • A rubber-sanding block
  • DA Sander or Orbital sander
  • Glazing Putty
  • Sandpaper
  • Hand Sanding Block

Sanding Fiberglass [7 Easy Steps]

Do you want what is the best way to sand fiberglass? If you want to know that, then you need to follow these steps.

Step-1: Keep the fiberglass to cure in the sun

When dealing with a piece of fiberglass, the gel coat has a thin layer on the surface. Gelcoat is an epoxy or resin-based material that you can use in the production of fiberglass molding components. You have to place it in the sun for 2 to 7 days before sanding to heal the gel coat. This procedure releases any air sacks from the gel coat that cause sanding and painting problems.

Why You Need To Cure Fiberglass?

The gel coat is used for the production of components of fiberglass to line the molds. Allowing the fiberglass to cure for a few days would allow any excess and unnecessary air that the gel coat holds in place to be released. It will significantly facilitate the sanding operation. This is just an easy way to get the job done more quickly and easily.

Step-2: Assemble the fiberglass components, if applicable

If your project consists of several components of fiberglass, assemble them before sanding or finishing. This enables you to sand between each part continuously, producing a smooth, consistent joint.

Step-3: Clean fiberglass with a wax and grease remover

Now you have the release agent on the glass fiber, and the only way to extract it is by applying a grate remover wax. This product is essential when dealing with a new fiberglass component, as the release material used to loosen the element from the mold must be removed. You can use wax and grease remover for car parts.

Step-4: Start the sanding with different sandpaper grit

It’s time to start the mission now, and you took all the necessary moves. You have to make sure that you are using the right grit-size sandpaper to sand before you start. The following should help you decide better:

Use 80-100 grits: When you start with fiberglass, and the surface needs rough peeling. You can use sandpaper with 80-100 grits. This helps to remove excess processing marks from the surface of the fiberglass.

Use100-120 grits: It’s time to sand a lot finer. When the markings are visible, it is time to smoothen the surface. It is advisable to take this step-by-step. It makes the work simpler and takes less time. This may not offer complete fluidity, but this is the strategy.

Use 150-220 grits: Now, you will have to choose the grit number according to the degree of smoothness you want. If you are searching for a completely smooth surface, you are looking for a higher grit.


The basic idea is to start small and progressively increase it. You should have a surface of fiberglass as smooth as you expected.

Since you’re a human being and you might file too deep spots by using the gross-grit sandpaper, you will need a glazing putty to replenish those spots so you can sand back into better views.

Step-5: Fill any low spots in the fiberglass

Use a fiberglass resin putty to improve the profile of low spots on the surface. Work the mask into the bottom place and sand it off before flushing the area with the rest of the floor.

Step-6: Apply primer to the fiberglass

If the coarse-grit sandpaper is done, you can apply primer to the surface of the fiberglass. This allows you to make smooth filing more effective.

Step-7: Do the finishing

You can now finally polish the fiberglass for smoothening the surface. One less coarse grit paper should be used for the initial sanding operation. You will again have your fiberglass under the sun for some time after this phase is completed.


Are you curious to know how to sand down fiberglass? You should use your hand for sanding while you are filing small fiberglass. Sanding with a device will minimize your power and versatility. Also, remember to apply with the same pressure, and you can stroke up and down to an even surface.

If you sand fiberglass, don’t forget that there is a high tendency for you to get any dust or particles in the air. You have to make sure you still have your respirator on.

How to Wet Sand Fiberglass?

It’s not at all challenging to wet sand fiberglass. You only need a bunch of wet sheets, such as 1000 grit, 500 grit, and 200 grit, for wet sanding glass. The follow these steps:

  1. 1

    First, get a bucket of water and a rag and try somewhere on a test site.

  2. 2

    Then start with the 1000 grit. If it doesn’t do well, then try the 200 grit.

  3. 3

    Keep the sandpaper in your water bucket moistened and washed away.

  4. 4

    If you think you need the 200 grit to get the oxidation off, then you’ll start with the rest of the hull.

  5. 5

    You can do the whole thing at once, or you can do it up to you. But after you have completed the 200, wipe it down, go up to the 500, and then the 1000 grit.

  6. 6

    A high-speed polisher with a particular compound or color is used to return luminosity after the 1000 grain. Then stick or wax.

Do You Have to Sand Between Layers of Fiberglass?

In layers, there is no need to sand. When a mat layer has healed, fibers are attached. If these fibers are not colored with a quick coarser paper when the next layer is added, they can produce tiny air bubbles.

What Kind of Sandpaper Do You Use on Fiberglass?

Suppose you want to strip previous materials such as gel coat, sand the surface with a grit of 80 or less. A low number of grits decide the coarseness of the sandpaper to extract materials and lacquers. Sandpaper with a number 200 grit and higher will be ideal for finishing, such as buffing, polishing, or scratch removal. A high grit of fiberglass sandpaper is proportional to how small the grains make the surface smooth.

Cost to Sand and Paint Fiberglass

The cost by linear foot. Professional paintings on the top can be between $100 and $400. It can be around $200 on average. Lower fiberglass sanding and painting is professionally between $15 and $100 per linear foot.

Final Words

Sanding fiberglass is just as essential as filing any content of fiberglass. You don’t want to finish with a rough, uneven surface, so it is necessary to sand.

To give your resin a smooth and even look, you must follow the steps already mentioned in the article. Since resins can be different in size and shape, a sanding block must be used. This should allow you to wrap your sandpaper and keep it well and comfortably to sand fiberglass properly.

Remember always that your protection and health are the first in any sanding operation. Sanding fiberglass with your open skin can cause itching. If it gets into your eyes, this could lead to some long-term complications.

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