How To Fiberglass a Mounting Block

Fiberglass Repair

This guide walks you through the steps of fiberglassing a wood mounting block to the boat hull.

This is an excerpt of a rough draft of a book I’m thinking about writing this summer. I hope this simple and useful guide inspires you to dip your toe into the wonderful waters of fiberglass work.

As you restore and modify a boat, you’ll eventually need a mounting block. This may be a support for a shelf or something to screw into in order to hang something. You should never screw through the hull of your boat, even if you’re sure the fiberglass is thick enough that it won’t penetrate the core material. This rule includes fiberglassed bulkheads, not just the outer hull. Screwing through fiberglass gives air and water a path to the core, which will eventually rot it.

Fiberglassing a wood block to the hull can provide all the structural strength you need without risking the integrity of your boat. In this post I will walk you through this simple fiberglassing job.

Sizing the Block

Storage Hammock

I needed to install a couple mounting blocks to helps hold up my saggy storage hammock.

If all you need is something to screw into, for mounting something light weight, then the block can be any size. If you need more structural support, like for a shelf, you’ll need a bigger block. The strength of the mounting block will be proportional to the amount of surface area glassed to the hull. A smaller block means less grinding and work overall. I prefer to use a 2” x 2” or 1” x 1” piece of hardwood. These can be found in any hardware store. I’ll cut the length depending on the amount of strength I need for the application.

Preparing the Area

Fiberglass Preparation

In this case, I only had to cut away the upholstery on the ceiling and roughen up the fiberglass with a grinder bit on a drill.

After choosing the length of the block, you need to prep the area to be worked on. If the area is particularly dirty, consider cleaning it with a degreaser or solvent first. Follow up with warm soapy water and give it a good scrubbing. The strength of any fiberglass job will depend on the quality of adhesion between the epoxy and hull. Any dirt and oil will prevent good adhesion.

Once clean, you’ll need to grind off the gelcoat and get to the fiberglass underneath. Epoxy will bond very well with fiberglass, but not nearly as well to the gelcoat. There are several ways to accomplish the grinding job with different levels of mess. For large jobs, a 4” rotary grinding wheel will be the fastest and messiest. It will reduce the gelcoat to a fine, electrically charged dust that will go everywhere and stick to everything. You’ll need to be careful to wear a facemask air filter, eye protection, and vacuum everything afterwards. Tarping off around the area to reduce the mess is a great idea.

Fiberglass Wood

Once all the prep work was complete, I mixed up the thickened epoxy and applied it to the fiberglass and the top of the wood block.

For smaller jobs, it’s better to use a drill or dremel tool with an aggressive steel grinding tip. This will flake the gelcoat off in bigger pieces and won’t produce nearly as much dust. It’s significantly slower and more manually intensive, but cleanup will be much easier. Either way, the goal is simply to remove the top, thin layer of gelcoat. You want to expose the fiberglass underneath, but not dig into it too deeply.

After the initial grinding, you may want to follow up with sandpaper, but that’s optional. If there is a great deal of dust, then wipe it away with a moist paper towel, but give the fiberglass plenty of time to air dry before mixing the epoxy. It’s also a good idea at this point to lay down masking tape if you need to protect against runs and drips.

Now that the surface of the hull has been prepared, it’s time to prepare the surface of the wood. Sand down every side of the wood block with 80 grit sandpaper. The objective is to simply clean off any dirt and oil it may have picked up in its travels. Don’t worry about shaping it, though personally, I do like to smooth out the edges of the corners before fiberglassing.

Applying the Epoxy

Vertical Fiberglassing

I then ‘smooshed’ the wood block into the fiberglass and held it in place with tape while I let the epoxy harden.

I rarely use cloth on these types of projects unless I need the mount to be incredibly strong. Instead, I just use epoxy thickened to the consistency of creamy peanut butter using 403 filler. Easy to paint on, but able to withstand the pull of gravity without running. This thickened epoxy provides plenty of strength by itself and cloth is usually not needed.

Once all the preparation work has been completed and wipe-downs have been given time to dry, it’s time to mix the epoxy. It’s important to take a deep breath and double check that all prep work is complete because once you mix the epoxy, the clock starts ticking. I prefer to mix epoxy in 1 or 2 oz batches. I rarely mix more than this at once, and it’s also hard to mix less than 1 oz. I use the smallest 2 oz measuring cup to mix the epoxy and hardener at the recommended 5:1 ratio. These small 2 oz measuring cups can be found at West Marine and most automotive and hardware stores.

It can be hard to accurately pour the epoxy or hardener strait from the can into the measuring cup. I always keep several 4 oz paper Dixie cups on hand. It’s easier to pour into these first and then from the Dixie cup into the measuring cup. Any unused epoxy or hardener can be poured back into the container. Just be sure to use separate cups for epoxy and hardener so they don’t mix!

Boat Hammock

The finished product now helps support the hammock and provide a plush head rest.

After mixing the epoxy and hardener in the measuring cup, I will generally pour the mixed epoxy into a Dixie cup and wipe out the measuring cup for re-use later. Also, when adding the 403 filler, the epoxy will expand, so a 4 oz Dixie cup gives extra room for this.

Mix in the 403 filler by the teaspoon. Stir in each teaspoon before adding the next. Be careful not to spill or inhale the filler as it is a fiberglass dust and very unhealthy to breath in! Stir it in gently to reduce particles from blowing away. Typically 3 to 4 teaspoons will get the consistency to that of jelly or peanut butter for 2 oz of epoxy. The epoxy thickens slowly at first, then quickly, so be sure to do just one teaspoon at time.

Once the epoxy and filler are thoroughly mixed and the desired thickness is achieved, paint the thickened epoxy onto the exposed fiberglass of the hull. Have a piece of tape ready. Squish the wood block into the epoxy and tape it in place. You can now paint around the tape. You’ll have to do a second coat after this first one hardens to get the area covered by the tape. On this first coat though, focus on the edges of the block where it meets the hull. Make sure there are no gaps by filling them in with the thickened epoxy. Use as much as you can, then throw away the rest.

Once the epoxy hardens, remove the tape and repeat the exercise to completely seal the block. After the second coat hardness, you now have a structurally sound, hermetically sealed wood block. You can screw into it without jeopardizing the integrity of your hull. Congratulations on your first fiberglass job!

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2 Responses to “How To Fiberglass a Mounting Block”
  1. CJ says:

    Thanks for the article.
    I’m adding an electric outboard to a dinghy (a Skipper 14), which means adding a battery, which means I need a method of securing a 22kg lump in a boat which could potentially be upside down. Do you think this technique would provide enough strength for a battery box mounting?

    • Chris says:

      I’m glad you’re finding some inspiration from this article.

      Yes, this technique shouldn’t have a problem supporting an up-side down battery. The strength is all in the surface area. If you want it to be stronger, increase the surface area. Overkill is always good. Grind down several square inches and glass down a piece a thick piece of plywood if you want a really secure mount. Get as many square inches of wood glued down as possible.

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