Joining Fiberglass Mold Part Halves

There are several ways to join fiberglass halves.  Fuselages, boat hulls and car bodies usually require joining of at least two pieces in order for the project to reach a point of completion.

With some applications, a sleeve can be built into the plug with will allow the second part to slip into which will result in a nice joint that can be epoxied together.  An application that comes to mind in this circumstance would be the deck of a model boat to the hull.  This technique needs to be planned well in advance since it requires integration into your plug.  This technique also is only applicable to those projects that won’t be visually hampered by a visible seem or joint on the final product.

Most of the time, in order to obtain a clean joint that can be filled, sanded and finished in a way that produces no visible joint on the final product, you will need access to the inside of the joined pieces in order to produce a clean union.  The best way to do this is to align the halves and use masking tape to hold the pieces together.  I like to use thin strips of resin saturated fiberglass mat that I can lay across the inside of the seem.  Depending on the strength of the bond that my project requires, I like to have about ½ inch of mat overlapping each side of the seem.  Once this cures, your joint is complete.  The only thing left to do at this point is finish the exterior.

Another method that I really like for joining fiberglass halves that don’t have to be water tight or incredibly strong is by using what I like to call “chemical applesauce”.  “Chemical Applesauce” is a mixture of resin and cabosil that has been catalyzed with MEKP that has the consistency of applesauce.  I use this mixture by taping the entire outer surface of the seem, making sure that the pieces have proper alignment.  I then use a gloved finger to apply this mixture to the seem line from the inside.  I use my finger to push and work the mixture into the joint.  This makes an excellent filler as well.  Once the mixture has cured, I can remove the masking tape and I am left with a strong, clean joint that requires only minimal sanding and finishing.

These are some of my favorite ways to join two halves of a fiberglass project.  I do have a challenging project on the table now that will require special consideration when joining.  I will keep you posted on my progress.

In my next post I’ll cover some issues on building plugs from foam.

Good Glassing to you,


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