Repainting over old paint
Dave and Andre have it about right -- but if you want to go into the subject a bit more, here are some links to some discussions in these forums of painting over old cracked or chipped paint, when you want the paint to last only a season or three before re-canvasing:
http://forums.wcha.org/showthread.php?t=5790 see pp. 2-3 of this thread
http://forums.wcha.org/showthread.php?7769-Painting-over-existing-paint&p=41339#post41339
http://forums.wcha.org/showthread.p...t-Restoration-advice-please&p=32358#post32358
http://forums.wcha.org/showthread.php?7775-Temp-repair-to-bare-spot-on-canvas&p=41357#post41357
http://forums.wcha.org/showthread.php?7619-time-is-not-on-my-side!&p=40689#post40689
http://forums.wcha.org/showthread.php?8564-Smoothing-Canvas/page2 starting at post 12, on Bondo spot putty
http://forums.wcha.org/showthread.php?6607-sanding-or-not&p=35286#post35286
My experience suggests that, at a minimum, removing loose, flaking paint, and then brushing on a coat or two of paint, either water or oil based, will get you through a season or two of paddling for only a couple hours of minimal work, until you have the time and the inclination to spend the time on a proper restoration. A coat or two of paint (for these purposes, almost any kind of exterior paint, water or oil based), over either bare canvas or crackly paint, will prevent most, if not all, leaks and give you a serviceable canoe. Removing and re-bedding the keel may be useful, but I have not had to do this on my canoe – at least not yet.
A useful product is Bondo spot putty – it is a completely different critter from the 2-part Bondo used to fill dents in cars, which is not waterproof. It comes in small and large tubes, fills small cracks, and dries rapidly.
Discretion being the better part of valor, it is wise to have a small roll of duct tape along in the event that some of the old paint/filler under your newly-applied paint decides to flake off. But even without a duct tape repair, the resulting leak will likely be very slow and will likely not interfere with a day of paddling. After all, a little water never bothered a real paddler.
More sanding, more spot putty, more primer will give you a somewhat better looking surface, with almost no improvement in function – at the cost of whatever extra work you wish to undertake – but while you will have a somewhat better looking boat, you will not be able to make a silk purse out of a sow’s ear. To get that “silk purse” look, you will need to replace the canvas and then do a proper fill and paint job, when you have the time and inclination to do it right.
My Old Town 50 pounder seen in some the pictures in some of the links above is in its 4th seasons of use with old canvas, chipped filler, crackly paint, and a few unrepaired cracked ribs and planks, for just a few hours of necessary work (and a few more hours just messing around with unnecessary painting of triangle designs).
