Step 6: Caulking
Now some people may be shaking their heads saying "you'll never be able to stop water from getting in without epoxy, fiberglass and lots of resin."
Actually all it takes is some cotton and putty. The putty will keep the water out until the cotton swells, the cotton will keep the water out until the wood swells
The simplest and surest way to do this is to just put flanel strips soaked in paint between every edge when putting the planking on.
But then you'd miss out on actually caulking a boat.
To do this you need some caulking cotton.
The easiest stuff for me to get is some 4-ply worsted weight 100% cotton yarn from walmart (peaches and creme brand), I've read about professional boatbuilders that use it, so if it's good enough for them...
Once you've got that some people will try to tell you you need a couple different size caulking irons and a special mallet and try to sell you some other specialized nautical boat building tools. For one small boat, especially a scow, you simply don't need any of that. You can caulk the whole thing with a thin flathead screwdriver, but since you need it for the putty anyway a metal putty knife will work better.
Take a peice of yarn a bit longer than the seam and start forcing it into the gap at one end. Push it about halfway through and then move across the seam trying to get the same depth all the way. If the gap is wider than the yarn simply fold the yarn onto itself and twist it a bit, then force that down the same as you did before.
After caulking the seams will need to be payed (or saturated with thinned, oil-based, paint). To do this get a small can of oil-based paint and thin it 10 to 20% with some mineral spirts or turpentine. Just brush this into the seams with a thin, short-bristled brush.
Another way of doing this would be to take a plastic "Dawn" dish soap bottle, a cork to fit it, and some 1/4 inch copper tubing and make a paint applicator. Just drill a hole in the cork for the tube, the tubing should be long enough to go almost all the way to the bottom of the bottle, put the tube in the cork and bend it about 90 degrees, then hammer the end down so there's just a slit for the paint to escape out of and file the edges down to about 1/16 of an inch thick. Then just fill the bottle with thinned paint and you've got a great applicator for future boats.
Then all the seams need to be puttied. Traditionally this is a mixture of whiting (calcium carbonate) and linseed oil or just boiled pitch. I'd try roofing caulk, the black tar-like stuff. I, completely ignorant of all this at the time of building, simply used wood putty. I may yet have to rip out all my caulking and start over, we'll see with time.
If you know you'll want to caulk the boat you can leave a 1/16 inch gap between each board when planking.
Due to some rough handling of the boat, I sprung a leak. To fix it I tried using putty made with whiting and linseed oil. The encyclopedia britanica describes whiting putty like so:
Whiting putty of a high grade consists of 85 to 90 percent whiting blended with 10 to 15 percent boiled linseed oil. Prepared putty should roll freely in the hands without exuding oil.
My first mix was probablly a little too gooey, but having fixed that I decided to recaulk all the other joints just in case. I found that it's a bit easier to mix in a shallow bowl or paper plate. you'll know it's the right consistancy when you can roll it in a ball freely but if you let it sit in your hand it sticks.