"Polish" doesn't carry over from epub to mobi

My Kobo takes epub; girlfriend's Kindle takes mobi. I discovered the "polish" feature via this forum, and it is great at doing something eBook designers should do automatically Adds back-cover detail to the front of the book, so we can see what we are getting into first.

I use the feature on all my books, then convert to mobi for the Kindle, but the polished info doesn't stay. Even if I deleted the orginal_epub file first.

I can easily copy the data from the epub, but am not sure how to get it into the mobi. When I select "Edit" for the book, it presents the epub version.