Hi @draxx,
A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Plaster is a very soft material that orbital sanders are too powerful for. I have used an orbital sander on plaster in the past, and trust me, it will rip right through it.
The best tool you can use for sanding plaster is a Drywall Sander. They are much gentler on the surface and will not rip through it in the same way an orbital sander will. If you were doing this sort of work all the time, it would definitely be worth the investment as they are brilliant, but I understand that it may be excessive for your situation.
Alternatively, you can use a Rocket 230mm Giant Sanding Tool or Hyde 225mm Radial Sander Plaster Tool with a painter's pole. These tools are designed for this purpose and are much easier on your back than hand sanding.
For intricate areas, use an angled sanding sponge.
Having done a lot of plaster sanding in my time, it is slow, dusty and taxing on your body, but to get things perfect, it is an essential step.
Let me know if you have any further questions, I'm more than happy to help further if required.
Jacob
