We HIGHLY recommend that you seal any of your wood projects that will be outdoors and keep them out of direct sunlight if at all possible. The Florida heat, humidity and direct sunlight can shorten the lifespan of your project.
The easiest & least expensive is the clear spray sealant below; you can find it at any hardware or big box store in the spray paint isle. Just make sure you do NOT get a polyurethane - it will yellow the paint, especially white paint. This one is my favorite and I prefer the matte look over a glossy look for my signs.
Let your project dry indoors for 24 hours then take project outside and spray evenly over the project. Don't forget the sides and the back (even if it's not painted). Cover in one coat, let dry for about 10 mins, then add another full coat. I personally put three coats total on my outdoor projects. Let dry thoroughly (for about 2-4 hours) then you can place your project outside.