Writer-based prose encompasses all the writing we do for ourselves and even all the writing we do for a reader before we really get to the place of putting ourselves in the reader's position. Therefore, some writer-based prose will never be transformed into reader-based prose, but some is on its way to that end.
Professional writers tend to keep journals, notebooks in which they can write down their observations or wrestle with ideas as they occur to them. These journals are for the writer, not for his or her later reader. They provide space for the writer to work out what she really thinks, and they become an archive of observations and ideas to which the writer can return in order to mine material for reader-based prose.
Journals have become popular teaching tools. Many teachers ask their students to keep journals, but unless students understand the nature of a journal and enter into it without fear of its being evaluated, they are unlikely to profit from the exercise. Journals are for the writers of journals. If we as teachers pick them up, we should do so knowing that we are peeking into the private musings of writer-based prose, and we should respect the nature of that writing, not expecting it to be something other than it is.