To be honest, if the bearings gritty, the damage is already done and is only fit for the bin. I run my with what is in them, if they got rubber seals, I will pop the seals and soak in car cleaner which breaks down compacted dirt, then into break cleaner and then allowed to dry. Grease is then used to pack the bearing and I use white diff grease.
I have found it more economical to buy cheap bearings and replace as needed than buying expensive ones and not getting much more life.
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