[chaley]

If it is important for you to remove some or all custom columns in exported books, create a new library that has only the columns you want then use copy to library to copy the books from your main library to this new one. Export the books from there. Delete the books from the new library once exported so that you don't get into future merge-books difficulties.

EDIT: the easiest way to create the new library is to create a new library at ... with "copy structure" true, then switch to the new library and delete the columns you don't want to publish. Of course, if you want to remove *all* custom columns then don't check the copy structure box.