Removing The Old Transom
First you'll need to remove the fiberglass covering on the old transom. Using a magic marker put a line around the transom about 3 inches in from the side and cut it with a rotary saw. Now peel the fiberglass off (it may require the use of a crowbar). Next, you'll need to remove the plywood or foam interior completely. This may require sawing, chiseling, and prying off. On the boat shown in Figure 1, it took an entire day to remove the wood in the flange by patiently chiseling it away.
Figure 2 shows what the damaged wood looked like as it was being ripped off.
Figure 3 shows how the transom was chiseled away in the area around the flange.
If you are planning to repaint the entire boat, simply cut around the corners of the transom. When you install new fiberglass, it will wrap around the corner of the transom and you will need to fair the topsides and transom. The boat shown here was cut three inches in from the edge of the transom to preserve the freshly painted hull sides. A new topcoat was only sprayed to the edges of the transom, making it hard to distinguish from the topside paint.
To make a template for the new wood, take a large refrigerator box and use the cardboard to make a template that will fit into the transom. Use the template to decide exactly where the transom is to be sectioned.
Making the Transom Pieces
Because of the flange on the boat shown here, a single piece of marine grade plywood could not be dropped into place. Consequently, both layers of the new plywood were cut into three sections -- a single piece from the boat bottom up to the well cutout, and two smaller pieces on each side for the first layer, as shown in Figure 4.
The second layer was made in three parts with vertical cuts at each corner of the transom. Each layer was installed separately in a dry run to make sure there was a good fit. See Figure 5.
When assembling all the pieces of a transom it is essential that all the parts go together quickly and easily or the epoxy might set up before the pieces job is done. After the parts have been dry-fitted and removed, all the pieces should be completely sealed by rolling them with an Epiglass epoxy HT9000 mixed with Fast hardener HT9001, as shown in Figure 6. The low-viscosity hardener easily penetrates the wood to seal it tightly.
This does two things. First, it make sure that water can not penetrate the plywood after it is installed, and second, it primes the plywood, making it easier to apply a second layer of epoxy.
The next job is probably the most miserable one in the entire process. The fiberglass needs to be ground back. First, put plastic sheeting under everything. That way the debris is contained. Next, protect yourself with a Tyvek suit, boots, rubber gloves, a respirator, and tight-fitting
goggles. When grinding back, make sure you cut back to at least three inches away from the cut. This gives the new fiberglass epoxy mix plenty of width for adhesion. On the boat shown here, grinding the fiberglass back took about two hours of miserable toil and the cleanup took about an hour. Figure 7 shows the pieces after they have been ground back.
Putting the Plywood Back
The next part of the job must be done quickly before the epoxy resin can set up. For the inner corners of the transom make up an epoxy mixture consisting of resin and hardener, glue powder, and filler powder that has the consistency of peanut butter. Fill an empty caulking tube with it. Squirt this mixture into the area around the inside of the flange to ensure that there are no air pockets in the transom. The first pieces of plywood (the outer sides of the transom) are then painted with a fresh coat of epoxy, put into place, and back-screwed through the inner layer of fiberglass to hold them tightly. Next, the inner vertical panel is coated with epoxy and pushed into position. The seams are caulked with epoxy before the panel is screwed to the inner fiberglass layer.
The next step is to coat the outer corners with wet epoxy and push them into place. A single screw can hold them until the other parts are in position. The final piece is the lower full-width piece, which may need some force to get it into place. Screw all the transom pieces together with 1 ¼ inch stainless steel screws every 6 to 8 inches.
Finally, put the cut section of the fiberglass transom into place. It, too, had been ground back to taper the edges for the new fiberglass. It is painted on the inner face with Epiglass epoxy and clamped into place. You should put a dozen or so screws through it to hold it tightly to the plywood until the entire transom has set up, as shown in Figure 8.
Once it is set, clean up drips and epoxy spills by grinding them back and apply fiberglass strips over the joints. Start with 3-inch wide strips and build up to 6-inch wide strips of 1708 or equivalent. Leave the fiberglass and epoxy to set up hard and grind the top back to a smooth finish. Once the transom is smooth, you can use either a mixture of epoxy and lightweight fairing powder or Interlux Interfill fairing and surfacing compound. To paint the transom, mask off the area and apply 2 coats of Epoxy Barrier-Kote 404/414 then sand it thoroughly with 320-400 grit sandpaper and then apply two coats of Brightside Polyurethane or Toplac. Figure 10 shows the finished job.
Original article from Interlux