Thoroughly mix 3 oz. of un-thickened epoxy and add WEST Low Density filler #407 until it is a little thinner than mayonaise. You want a mixture that thin enough to flow through a caulking tube without running out of the laps. Pour the mixture into one of the empty caulking tubes and insert the plastic end cap. Using a sharp knife, cut the tip of the caulking tube nozzle so that the hole is as small as you can get; less than 1/8" in diameter is good.
Being careful to avoid getting the filler mix all over the planking, run a small bead of the epoxy mixture along the top of each lap on the inside of the hull. Also run a bead along the keel joint, leaving 1/2" on either side of the keel stitches. After you have used up all of the epoxy mixture in the tube, use the end of a Popsicle stick to put a radius in the epoxy mixture, making a fillet. Try to get a continuous bead of the filleting mixture along the whole length of the lap. Using a putty knife or the sharpened end of a Popsicle stick, scrape off any excess epoxy mixture from the plank's faces into a small cup. Use this mixture to fill in any voids in your fillet.
Mix up another 2 oz. of a non-sagging epoxy/low-density filler mixture, and put it into the caulking tube. Fillet the inside of the stems, using a tongue depressor as a tool with the proper radius. It's really hard to work up in the stem area, but don't worry, there's an easier way. If you have trouble filleting the stems, try pouring a stem plug. If you decide to pour a stem plug, don't worry about doing a great perfect job on the stems at this point: all you want to do is get enough of the epoxy mixture in the stems to hold them together until the stem plugs are poured. Clean up any extra epoxy mixture, especially in the stems where it will be hard to sand later, and double check the hull for twist, mis-aligned stems, or anything that doesn't look just right.