ambiguity - Present perfect or past simple?

I know the basic rules about using PP and PS (like specified time in the past etc.) but in some cases, it is not clear to me:

I have finished painting. - present perfect, I just announce that.

I have finished painting for today - not sure here? I say that today I will not paint anymore but still I feel PP could be possible. What tense should be here?

Answer

If the function is to identify when you finished painting then you might say
I finished painting today.
I finished painting for today at four.

But if the function is to identify the current state of the painting then you might say
I have finished painting.
I have finished painting for today.

These mean you are in one of the following (respective) states:
finished for good (for the indefinite future, for the current painting job)
finished for today

You could also say
I am finishing up painting for today.

Note that "painting for today" is the job in focus, not "painting (until completed)", so we are talking about entering the the state of "finished for today" rather than "completed painting".