I’ve heard (though I don’t have any experience with them first hand) that PTR-rollers aren’t
very effective in doing what they are supposed to do. They basically “transfer” dust particles from one point on the film to another.
See here: http://www.film-tech.com/ubb/f1/t011338.html and
In order to get good results from the scanner we’d also need to figure out a way to clean the film properly (unless using new stock, of course). Many post houses are using a ultra-sonic cleaning process to clean the film before scanning. Of course this isn’t directly related to building a scanner, but nontheless an important part in the transfer from film to digital.