Mersenne, the sealed bearings may not survive all that well if they are exposed to water. The inner and outer races of each bearing will rust at their ends, and then the rusty races will tear up the plastic side-seals of the bearings. I think it is more usual to use a grease compartment that is lubricated from time to time with a grease-gun, via a grease nipple. If you have a close fit or a labyrinth seal on the end-plate where it runs on the axle, it will probably only need a couple of strokes of the grease gun each couple of years. That will re-fill the grease compartment and push a tiny amount of grease through the labyrinth seal. Any dirt present will be pushed out with that grease.
