In my view, none of them is correct. Maybe other readers might have a different perspective, but to me, 'as... as...' is a fixed comparative expression and if you use the second "as," you also need the first:

I am not as good a painter as she is.

Technically, the "is" can be truncated off, leaving you with, "I am not as good a painter as she." This, however, would only really be found in a rather posh form of old-fashioned British. The underlying reason is that most English-speaking people are somewhat confused about what is correct here, and they are unsure about whether they would need to switch to "her" if the "is" was left off. Just people with very good old-style British educations collectively knew enough English grammar to be able to converse comfortably using correct pronoun complements with the verb "to be." (They were perfectly clear on what to do mainly because they had also studied Latin.)

"I am not as good a painter as her," is grammatically wrong, but in a sentence with "not as good a (something) as" in it, this grammatical error is obligatory schoolyard slang for anyone under 18 in Canada and the US, and it would seem pretentious for any kid to use the correct phrasing when talking to friends. "I'm, like, not nearly as good a bubble gum bubble blower as her." In movies, therefore, you will often hear this kind of usage. Continued usage of the mistake in adult life is also written into movie scripts for criminals, cowboys, hookers, barmaids (of the kind that call customers "honey") and so on. "Sorry, honey, you're just not as 'generous' a lover as him, know what I mean?"

The closest correct thing I can think of to your examples is the somewhat strangely worded, purely conversational (not written English except dialogue):

"I'm not such a good painter -- not like she is."

There is also an old-fashioned "so...as..." comparative, as in "I am not so lucky as she is," but I can't imagine anyone saying, "I am not so good a painter as she is." It would seem very awkward.