Is it possible to make the plaster surface in the bathroom waterproof sealed?

Hello everyone,
We are currently in the final stages of renovating our bathroom.
We only want to tile the area behind the toilet and the wet cell.

Generally, there is also a tile backsplash above the bathtub, but we don’t really like that.

Throughout the whole house, we have only plastered and painted the walls. The same should happen with the walls in the bathroom so that it visually matches everything else.

Now the question
Can the surface of the plaster in the bathtub area be sealed waterproof and water-repellent without impairing the paint?

Red band plaster was used throughout the house, and the same is planned for the bathroom.

Thanks in advance for your answers.

Gypsum has NO place in the bathroom!!! Usually, lime cement is used in the bathroom. Gypsum molds in the presence of moisture and there is NO paint that is diffusion-proof. Therefore, it will lead to mold damage in the bathroom in the long term.

Good to know. In the guest bathroom, the areas that are not tiled have normal gypsum on the walls. When asked at the hardware store, they said it would not be a problem :/ There is also a shower in the guest bathroom that has been used for about a year because the master bathroom is not finished yet. I hope that won’t cause any issues now or in the future if someone showers there occasionally. So basically, for the bathroom upstairs, you would apply lime-cement plaster to the walls and that would be fine? And the row above the bathtub should definitely have tiles if you can’t otherwise make the wall waterproof against possible splash water?

With lime-cement it doesn't matter, it is waterproof. It is also used for exterior plaster. You can simply paint over it. However, I would still put a row of tiles because it makes cleaning easier when scrubbing the edge of the tub.

It has now happened.
Stick Stotex, paint Sto opticryl. That would then be similar to the tile in terms of tightness and scrub resistance. Karsten

And the lime-cement later looks exactly like the surface of the plaster or is it to be processed that way?
In other words, apply it with a trowel and smooth it with the fan.
Then paint it and the splashing water won't cause any problems?

If that's the case, we would definitely not put tiles on the bathtub. It's not bathed in that often to require any special cleaning.
You usually don't clean normal walls either.

The stotex is the fiberglass wallpaper. That is not desired, but thanks for the tip anyway.