Tire sealant has been around for a good many years now and has certainly improved greatly in that time. What started as a thick sludge suitable only for a lawn tractors or wheelbarrows has now progressed to a foaming lightweight liquid suitable for a race wheel tubular. When I was mountain biking regularly in England I had great success avoiding punctures by using a bottle of the green slime in each tube. When the tires wore out and were removed from the rims there had been so many punctures sealed that the tubes had welded themselves to the tires, made me a fan of the stuff for life. It is still sold today and is still Martian green and still works great in a tube setup.
Basically all the sealants work in the same way. When the casing is breached air rushes out through the hole taking the sealant with it, as the sealant flows through the hole, particles in its formula seal the gap. Although the principle is the same throughout the range of products on the market we have found some of these potions are more successful than others at getting the job done.
For a comfort on hybrid bike or even a fat tire mountain bike running regular tubes Slime is still hard to beat. It is easy to install in a Schrader tube and, with a little patience and a removable valve core, it can be used on skinny valves too. Although it can be put in road tubes I find that it does not work quite as well under high pressure. Although Slime is about the beat of the bunch when using a tube, making the switch to a tubeless system can offer up a whole new set of possibilities.
When using a tubeless system, such as the one offered by Velocity or the similar Stan's method, we prefer to use the Cafe Latex brand of sealant and the Stan's sealant. When putting a tubeless mountain setup together we prefer the Stan's sealant. This stuff works great on big tires, it seals quickly almost any puncture from thorns to nails and helps seal any areas around the valve and along the beads. It will not seal a cut or slit, none of the brands will, and like other makes, it does lose its effectiveness over time. Any punctures sealed stay sealed but after around 3 months the mixture dries up considerably and ceases to work sealing new holes and needs replacing.
When it comes to tubeless road systems we prefer the Cafe Latex brand. This stuff is great on high pressure skinnies and seals quickly and permanently any punctures. Like the Stan's it also helps seal values and beads however we do find that it tends to keep its effectiveness a little longer.