USAMARINE Web Group - Captains' Corner Boatbuilding Tips

Core Repair - (Transom Damage)

While this tip was originally addressed to a badly damaged transom, the overall technique is consistent with other repairs that might be needed, where an encapsulated core is involved.

Remove all damaged material.  The fastest way is to probably go after it with a reciprocal saw and cut back a little beyond the damage into what seems to be good stock.

Feather the outer layers of fiberglass by three to four inches away from the cuts.  The glass thickness at the cut will be zero, 3-4" back will be 100%.

I'm assuming that the core material is marine plywood, (most small boat transoms are). Using an appropriate number of layers of marine ply, build a blank the same thickness as the original core using a marine laminating epoxy resin.

Coat each surface first with un-thickened epoxy, let it soak in a bit and add a second coat.  Now thicken another batch of the same epoxy with high density filler, (milled fiber will do fine also).  Trowel the thickened epoxy on with a notched trowel.  The cheap plastic one are great and disposable.

Mate the pieces together and screw them together with all weather decking screws. Don't over tighten them, you don't wont to squeeze the joint dry with epoxy, the way you would with wood glue.  After the epoxy goes through it's initial cure, (about 12 hours at 72o), remove the screws.  After a 24 hour cure, cut the slab to fit your cut out in the transom.  Carefully epoxy this into place, let it cure for a couple of days, sand everything down well, and you'll be ready to glass it up.

Note that "splining the joint" at the time of installing the new section will greatly reinforce the joint.

Glassing it in:

Start with a layer of 1 1/2 oz. mat, followed by layers of 1708 bi-axle until the same thickness of the original is matched.

Overlap the the glass all the way over the feathered area.  Each successive layer can be cut back a touch so as not to build too high of a ridge over the old glass. This can all be done in one glass up.  You don't need to wait for each layer to cure.  Any layers that cure more than 24 hours must be scrubbed down with soap and water, dried, and sanded before laying another layer.

Let all of this cure for about a month, to assure that the bulk of the shrinkage has taken place, sand down the whole thing, add a couple of final layers of 1 1/2 oz. mat, cure it for 48 hrs, sand again and start the fairing process, using either a commercially prepared fairing compound, such as Awlfair, or by mixing a fairing compound with epoxy, such as West 410 micro-light and a touch of cabosil to keep it from sagging.

After fairing, you can prime with something like Awlgrip High Build primer, and paint. Just about all of the marine 2 part finishes will bond well with the Awlgrip primer. The whole repair process, start to finish, should span about 6 weeks for someone with a fair inclination for DIY-ism.

Have fun! Feel free to download and print this page, but please don't use it on a website without linking it to

Capt Patrick McCrary

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