How to Replace Transom Fiberglass

This rear panel transom is covered by a protective rubrail.

A boat transom is a panel located on the rear of the hull of small boats, often containing a small door. The transom is built from a strong, sturdy composite of wood or foam layered on both sides with fiberglass. At some point the fiberglass layer will need to be replaced, usually after a modification to the existing transom. With a few tools and the right process the replacement can be completed quickly.

Things You'll Need

• Measuring tape
• Duct tape
• Work gloves
• Safety goggles
• Respirator
• Rotary tool
• Diamond cutting disc
• Putty knife
• Flat head screwdriver
• Sandpaper (150-, 220-, 400- and 600-grit)
• Sanding block
• Tack cloth
• Fiberglass tape
• Epoxy resin
• 1-½ oz. fiberglass mat
• 1708 bi-axle fiberglass cloth
• Gelcoat
• Catalyst
• Heatgun (optional)

Instructions

1. Measure and mark the edge of the transom 1-1/2 inches from the perimeter of the transom to serve as a guideline for cutting the fiberglass. Cover the line with duct tape to keep the fiberglass from splintering while cutting.

2. Put on long sleeved impermeable clothing, work gloves, safety goggles and a respirator to provide protection from fiberglass particles.

3. Turn on the rotary tool and cut through the center of the duct tape. Start from an upper corner of the transom and cut through the duct tape and fiberglass completely around the perimeter. Do not cut into the core material.

4. Use a putty knife to slip beneath the edge of the fiberglass skin. Holding firmly to the duct tape and fiberglass skin, pull steadily upward on the fiberglass, working the putty knife as you go to remove the skin from the core. If you can't get enough purchase beneath the skin with the putty knife, use a large, flat head screwdriver instead. After removing the center piece of fiberglass, remove the edge pieces using the same process.

5. Smooth the core, removing the resin left over from the fiberglass skin with a piece of sandpaper placed onto a sanding block. Use sandpaper to sand away the adhesive residue left on the core from the resin applied to hold the skin in place. Wipe the surface of the core clean using a wax-impregnated tack cloth.

6. Apply fiberglass tape around the perimeter of the transom to seal any joints on the transom.

7. Spread a layer of epoxy resin paste onto the surface of the core, covering the fiberglass tape as well using a brush. Place a 1-½ oz. fiberglass mat onto the layer of resin. Brush a layer of epoxy resin onto the mat, covering it completely then place a layer of 1708 bi-axle fiberglass cloth over the mat. Place alternate layers of mat and cloth, with resin between each layer, building up a fiberglass covering that's as thick as the original that you removed. Allow the layered fiberglass 24 hours to cure.

8. Apply gelcoat to the surface of the fiberglass to finish the covering. The gelcoat is the smooth paintable finish that you see on transoms which covers the weave material of the fiberglass. Mix the gelcoat with 20 drops of catalyst per ounce of resin using a wooden mixing stick. Brush the fiberglass with the gelcoat, building a layer about 1/32 inch thick. Wait an hour for the gelcoat to cure.

9. Sand the surface of the gelcoat until it's level using 150-grit sandpaper on a sanding block. Use 220-grit wet dry paper and sand the surface, spraying the surface with water as you remove the remnants of the sanding. Switch to a 400-grit paper then 600-grit, smoothing the surface. Use a rubbing compound to buff the surface of the gelcoat.

Tips & Warnings

• Soften the adhesive holding the fiberglass in place by directing the heat from a heatgun across the fiberglass surface.

References

• Tap Plastics: How to Repair Fiberglass
• Hydrostream: Transom Replacement
• Bertram 31: Core Repair

Resources

• TransomRepair: Basic Repair Techniques