Should the roughcast be sanded smooth?

Is it possible to smooth out a rough plaster like the one in the picture with a long-neck sander? Or do you need a different device for that, or is it even possible at all?

Thanks!

Depending on the type of plaster. If it is gypsum, you can scratch it with your fingernail and therefore easily sand it off. Otherwise, a concrete grinder is probably needed. However, it smooths the plaster less and instead removes it completely. Gypsum plaster (I call it pick plaster) can also be easily smoothed with a rabbet or aerated concrete/ETICS sanding board. It produces much less dust and the dust falls directly down instead of swirling around in the air.

So I think you can't scratch in there with a fingernail, I would have to try again. So if it is not gypsum plaster, it looks bad for sanding smooth, did I understand that correctly? I just don't want to have to replaster.

I would try to chip off the plaster at one spot as a test. If it is that hard, you can definitely still remove it with the concrete grinder (device costs about 100-120 euros new), which is no more work than sanding. The concrete grinder also sands it off, but you really have to be careful to work evenly, otherwise the surface becomes very uneven. That's why I said completely removing it is probably easier. However, then you also have to replaster. Another option: only work on the burrs with the electric scraper. That should work, but will surely take some time. You need the attachment for hard materials (there is also one for soft materials, which is flexible and gets damaged quickly on hard surfaces). These disc sanding machines probably won't work if the plaster is very hard (so not gypsum). Other alternatives: smooth filling, but that will likely require a lot of filler. Or stick gypsum fiber boards on top with adhesive, but then a new surface treatment (filler, primer, decorative plaster/wallpaper) is needed again. The decorative plaster can be easily rolled on with a paint roller, so you can do that yourself easily as well. I would probably try the concrete grinder if gypsum boards are out of the question. Depends on the area.

Ok thanks, so filling with filler putty is out of the question, and putting plasterboards on top again is also not an option. A concrete grinder would then be something like in the attachment?

Yes, but that is already a more expensive model. The ones for 100 euros do the same. Otherwise, you can also rent them.