How to Build a Fiberglass Window Box

What You'll Need
Fiberglass Mat
Epoxy body filler
Sandpaper (low and high grit)
A mixing bowl
A piece of wood (to mix with)
Putty knife
Screws and window

A fiberglass window box is a good choice, since fiberglass is a very reliable material. You can use fiberglass for any shape and size. It is ideal for outside purposes, because it can withstand tough weather. This article will show you how to use fiberglass to build a simple yet reliable window box.

Step 1: Molds

Fiberglass molds can be purchased at DIY stores, but is better to create your own mold, since your project may have unique measurements. Fiberglass molds aren’t difficult to make, and can be used over and over again. You can build a whole form, i.e. build a window box mold, or build separate molds and than attach the resulting fiberglass together. Make sure that the form is smooth, because the fiberglass will imitate any defect present in the mold.

The best material for a fiberglass mold is wood. Wood is quite easy to smooth out, and it has the advantage of keeping its shape. Alternatively, you can use cardboard to create a form. Cardboard is very useful if you manage to find a box with the intended measurements. This saves you the hassle of modifying or building a box to suit your particular needs. The most important thing is to make sure that the corners are well built. It is advisable to apply a good quality epoxy body filler with your putty knife, so that the fiberglass doesn’t stick to the form. The body filler comes with a hardener, which is applied and mixed together before applying, so it is best to work on small areas at a time, as it will harden when dried.

Step 2: Fiberglass Strip and Resin

Now, apply the fiberglass strips and resin into the form. Lay them properly and evenly. If necessary, you can choose the cross-layer method. The cross-layer method is ideally used for creating largish window boxes, intended to hold the weight of a considerable amount of wet soil. This is done by criss-crossing the fiberglass strips onto each other, while building up the base and edges of the box. This method makes the window box heavier but stronger.

Step 3: Edges

Next, cut off the fiberglass strip's edges. Always make sure that they are undamaged and neat. If you want, you can add some more resin to thicken the edges. Once you are done, leave it to dry completely. This may take any where from 10 to 15 hours.

Step 4: Sanding

First, sand the edges using low grit sandpaper. Afterward, re-sand them using high grit sandpaper (360). It is important to make sure that the edges are sanded properly.

Step 5: Removing the Form

Don’t be too hasty in removing the form, otherwise you may cause damage. Remove it by pulling it carefully away. The body filler applied earlier should speed the process. Tap the form gently on the edges with a rubber mallet. Once removed, clean the box with a damp cloth, and use a low grit sandpaper to smooth out any small imperfections.

Step 6: Brackets

Attach 2 or 3 L-brackets on the back of the window box. Use a small drill bit, to drill the necessary holes. Attach the L-brackets with stainless steel screws.

As you can see, it’s not that difficult to build a window box. An important thing to remember is to work in a well ventilated area. This will help the fiberglass to dry quickly.