To diverge slightly since you mentioned trailer hubs....
On the front grease caps, I have used Teflon tape to completely seal the cap from creating a grease film slime on the wheel and hub. I used 1/4" wide Teflon tape and carefully positioned the tape up near the flange of the cap on the OD. You want the tape to stay far away from the brim of the cup. Doing that will prevent strings of tape from getting into the wheel bearing/grease area as you install the cap. When pounding the cap on, the Teflon tape scrunches up and forms a nice little gasket between the hub and the flange of the cap.
No more grease slime!