Plastering a Corbel.. how the f do I plaster this thing..

Im finishing my last room out of 4 this weekend but Ive got to tackle a corbel next. Its rounded and been done in a mix of lime and gypsum over its life. It looks like its going to be a f**k*r to redo..

any advice on how to get a smooth finish on this? Im not sure a trowel will work. Literally best options using my hands at this point.