If you're going to create things out of cold porcelain, you have to be aware that even when fully dry, they're still not resistant to damage. They can be easily scratched and water will melt them (bear in mind that most of the ingredients used in cold porcelain are water soluble). If you want them to last, you have to use a sealant.
There are many sealants that you can use, but there are three easily accessible (they can be bought in any hardware store) products you can use, namely waterproof primer, polyurethane floor varnish, and clear gloss varnish.
All three do a good job when it comes to protecting your work from weather damage (and scratches, to some degree), but they differ in a few things, and you may want to read on if you want to know which ones will work best for your project: