I have been needing to do a full service on my rear derailleur for the longest of times now.
My poor excuse for postponing the task, has been the lack of ownership of a pair of quicklink pliers to split the chain. Fiddling around trying to push the links together by hand or with regular pliers or bits of wire is such a faff.
So this weekend I bit the bullet, rummaged around in my box of cutoffs for suitable material to make a proper tool. An hour later, with the aid of a hacksaw, a file, a drill, a pop rivet and some rubber pipe, I had turned a length of 15mm box section into this magnificent weapon.
Chain off, I could then get on with the actual mission. Remove, dismantle, clean, re-grease everything in/on the derailleur.
Then reassemble and re-install the rear derailleur. Jobs a good'n.