Rot Doctor


Subject: Old Lapstrake Hulls
Date: Fri, 10 Nov 2000

Dr.Rot, I am about to flip my 1939 Lyman mahogany lapstrake hull and remove all the old paint and compounds used without success to keep the water out.The boat is kept in a boathouse (damp environment) all year and used a lot. What would the proper procedure be to apply your product and which compound to fill the laps would you recommend?
Thanks, Jim D.


Well, that’s an old Lyman! About now I wish we had one like it, instead of our 50′ 1889 tug.

If you’re going to wood the boat, then certainly a coat of the CPES (Clear Penetrating Epoxy Sealer) on the outside would be very beneficial…help preserve the wood and certainly make your paint last a lot longer.

As for filling the laps, we’d recommend a polyurethane sealant, applied directly to the CPES-treated surfaces after the CPES has had a couple of days to begin the cure. We have just completed a series of tests on sealants applied to CPES-treated wood, and frankly they all (forget about silicones!) adhere pretty well. We do believe though that the polyurethanes are particularly adhesive, and our tests tend to confirm this.

But which polyurethane? 3-M’s 5200 is hotly popular right now, and it is an adhesive as much as it is a sealant. Good stuff. But the trade-off you make with the 5200 is that it is much less flexible, and this fact alone makes me a bit nervous. The 3-M 4200 is better, but more and more I am seeing their “Quick-Cure” version of the 4200, and our testing shows that it is less flexible than the standard 4200. We also tested an industrial-grade polyurethane sealant, Dymonic® FC, which we sell. It showed all the tough adhesion of the 4200, but retained a greater degree of flexibility. We could get this for you if you wanted, at around $8.00 per gun cartridge, in a variety of colors, including white.

Polyurethanes can also be painted over.

For sealing the laps in a Lyman, I would suggest making the open areas as clean as close to wood as possible, applying the CPES to the whole hull, waiting a couple of days, and then begin the sealing. Assuming the laps openings are 1/4″ or less, then I would just draw or push a bead along the seam, and when finished go back with some sort of round-tip instrument dipped in soapy water and draw everything smooth. You can use your finger to do this if you wish. Excess can be removed with Xylene or Tolulene before sealant cures, although this shouldn’t be necessary in your application. After it cures, it has to be shaved away from the wood. If you are using a standard polyurethane, such as the Dymonic FC, you get a tack-free surface in 24 hours in under 60–70F temperatures, and 72 hours at any temperature. Final curing takes place at 1/16th″ per day. Paint can be applied after the surface is tack-free.

So there you are, a nutshell version of what we know about sealants. Most of what I have said would be concurred with by anyone experienced with sealants on boats.

Come on back if you have further questions, or if we can be of further help.