120 grit is not a typo. That's plenty smooth. It does not need a mirror finish. I have installed several heads treated in that manner without any issues. I use the paper from an 8" wide roll so I do not have any seams. You could finish it finer if you wanted to but it is not necessary in my experience. You would want the 1/4" steel supported solidly and flat also or it will flex but if true and flat it will work. I solidly support the piece of polished granite I use to keep it from flexing. I lube the paper with WD40 or the like and use a good technique of keeping the head flat when moving it and altering the direction.
...BTW your Bieber avatar is awesome.-Malone
I need to get some pics of the ugly beast up here soon.