You might be able to do a sulfamic acid or 0 TA treatment.
If you are planning to drain, you can do a zero TA or sulfamic acid treatment to see if the surface evens out.
Zero TA is very aggressive, but not as aggressive as an acid wash.
If not, you can try using diamond grit sanding discs to sand down the plaster.
Note that sulfamic acid and zero TA have a lot of risk like damaging any metals that are exposed to the acids.
Do your own research before choosing a method of surface cleaning.
Never drain unless you are sure that it is safe to drain.
You can acid wash, but it is almost never the best choice.
Acid washing might improve the appearance, but it will significantly etch the plaster and make it very rough and it will significantly reduce the remaining plaster life.
For acid washing, probably 90% or more people regret acid washing while less than 10% are happy with the results.
It is likely that the plaster is due for replacement.