Rust coming through paint in ceiling

We have a vaulted ceiling in the open lounge dinning area. When the plasterer did his job he used a metal angle along the V down the middle of the room. Now after 10 years spots of rust are coming through the plaster and plaster is cracking off . About 4 years ago, between tenants, I did some patching and touch up but now we are moving back to the house and I will be painting inside before we move in. I want to do a more substantial repair and was wondering my best approach.

Any ideas?


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