Rather than applying the sealer with a brush, some people choose to use a garden sprayer as a way to deliver the sealant to its target.
You don’t want to try a paint gun because a sealant will clog the tip very quickly. A garden sprayer may also get clogged from time to time, but they are a little easier to clean. If you go with this option, I would recommend that you use a brush for the railings. The garden sprayer should work fine for the deck surface, but using it on the railings will just cause too much overspray.