A recently posted question, with 11 down-votes, rightly closed but still viewable, had been edited---doubtless with the best of intentions. But I would rather have seen it unedited, because it then would have been quicker to make an assessment of the post. If an editor adds a skin of quality, the reader may pause to wonder whether there is really something in it. I don't think that the editor believed the post to have much underlying merit anyway, or he would have argued for retaining it. Thus, I would ask potential editors to think first whether editing at all is going to serve a useful purpose.
There are two kinds of low quality posts:
If a question can be cleaned up to become a good question, users are encouraged to edit the question and upvote it. Edits made to a question can be seen in the edit history for that question.
If the post is beyond redemption, vote to close, or flag it as "Low Quality."