How to Stop Terracotta Pots from Cracking 

I use a rubber sealant in spray can form. I've heard some rubber seal brands come in a can to brush on. I found mine on road trip through New Mexico. It's called: "Tom, Dick & Harry's World's Finest Flexible Rubberized Sealer"

In my experience, I've had pots that were fully sealed all around end up cracking and spalling too, so I'm honestly not sure if it's better. Plus, depending on the plant that's in the terracotta pot and your conditions, you may actually want the pot to be porous. I grow plants that are sensitive to root rot in terracotta. I find that my herbs like sage and rosemary do much better. You should, of course, always also use quick-draining soil, but the terracotta is extra helpful, especially in the rainier times of the year.