When I had my trailer frame made I speced that the hubs had "Bearing Buddies" on them. They are the grease fitting that is on the end of the spindle. A few weeks ago I went to squirt some grease in them and found water inside. They come with a little rubber plug to cover the hole. These plugs have a lot of play in them, allowing water in. I tore the hubs apart and repacked, by hand, the bearings. I put some grease around the edge of the rubber plug. This seems to work, but I wanted something more subsustantual. I picked up some rubber caps from HD in the plumbing dept. They fit tight and will keep water out.
Wow, look how clean those wheels are with grease in the bearings. Mine are not. That is why I like the black wheels. Though thanks for sharing the idea.