There's a tire sealer called "Slime" that works like a charm. On my lawn tractor, the sidewalls on the tires were horribly cracked and the tires were obviously shot. On a whim, I bought a bottle of "Slime" and poured it in according to the instructions, and the horrible tires actually lasted another season with no trouble!

Slime is a green, gooey substance, and some of it actually seeped out of cracks in the sidewall, but it quickly sealed the tires. I've since used it to fix leaky tires on a wheelbarrow, hand truck, and, most impressively, on my Ford 640 tractor. Works like a charm. Which is good, because I used to have endless trouble with slow leaks.

On Slime Web site, they claim that the stuff never hardens and can be washed out with a hose if you want to repair the tire for real. I haven't tried this, but I believe it.

There are several variants of Slime: for tubeless tires, tube tires, low-speed tires, car tires. You can even buy bicycle inner tubes with Slime pre-installed.

You can find this stuff in hardware and automotive stores, and online. This stuff is great. It's the kind of thing you want to buy on spec and keep on hand.