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:
1. Waterbased Waterproof Primer - this is normally used for walls and tiles and can be had for cheap, but they're rarely sold in small containers. The good news is that you can just buy a container and it will last you a long time (I bought mine a year ago and the container is still 75% full.) The label states that these can last for 3 years when stored in an air tight container, so it's still cost effective if you plan on doing this for the long term. This stands out among the three because it provides a matte finish (whereas the other two provide glossy finishes)
|Waterbased Waterproof Primer|
2. Polyurethane Floor Varnish (Top Coat) - easily the most expensive of the three. It's commonly used for stone/marble flooring and wood furniture. It comes in two cans: a small one and a big one that you have to mix before using.
PSA: if you only plan on using a small portion, don't mix everything. Just take a portion from each can and mix them in a smaller can (make sure to follow the proper proportion - in the case of the brand I have here, it's 3/4ths from the big can and 1/4ths from the small can.) The reason for this is once you mix the two, they will start to harden within a day even if you keep the container sealed airtight. I had to learn that the hard way because the label didn't provide any warnings.
|Polyurethane Floor Varnish|
3. Clear Gloss Varnish - this is normally used for woodwork. It's the cheapest sealant out of the three (you can buy three cans for the price of one can of polyurethane), and provides a good middle ground. The result is not as sturdy as polyurethane, but still tougher than waterproof primer. The output is glossy as well and helps fill in cracks in the paint. It's advisable to apply it thin first and let it dry overnight, then apply another coat when it's dry. The second coating will dry faster.
|Clear Gloss Varnish|
It's also important to remember that polyurethane floor varnish and clear gloss varnish are both flammable, and that all three are toxic when ingested so take the necessary steps to keep them away from heat, children, and pets.
Note: I'm not promoting any brand. I have these brands because they're the cheapest in the store (or the only available brand.) Feel free to try other brands, and let me know if you find something that works better :) .