Playing with Tenses - Past perfect vs Past perfect continuous.

I've been learning English for a long time, but I never studied grammar.__So by I am now I'm taking formal lessons. But However, I'm a little blocked stumped today.

I made this example:

Past perfect - “Manuel had repaired many cars before he received his mechanic's license.”
Past perfect continuous - “Manuel had been repairing many cars for years until he received his mechanic’s license”

Now I need to do the same with this:
Past perfect - “She had never seen such a beautiful painting before she visited the Sistine Chapel.”
Past perfect continuous - (((????))))

I tried one, but my teacher told me that it was wrong, it was not past perfect continuous.the grammar.

“She had been seeing many paintings, but nothing as beautiful as the one in the Sistine Chapel.”

What you wrote means that you are comparing paintings with the chapel itself.  I don't know what is past perfect continuous mean exactly.  I had to look it up.  However, there is nothing wrong with what you wrote except for the highlighted corrections.

"She had been seeing many paintings but nothing as beautiful until she saw the one in the Sistine Chapel."

http://www.englishpage.com/verbpage/pastperfectcontinuous.html