I have a little 1x30 belt sander and it rounds out plunge lines quite a bit but I like a nice sharp plunge cut so I go over it with a flat file. After I do that I can't get the file marks out with sandpaper. Is there some trick to do this?
Read and study Nicks WIP. There is so much good info in there he should charge for it. He shows his sanding sticks and how he cleans up his plunge cuts. I use sanding sticks and techniques just like Nick and I get Great results. Take the time to go through it and you will be glad you did.
I don't know if you can do it with a 1x30. You need flexible belts. Generally if you are doing this on a belt sander, you track the belt off the side of the platen about 1/8" and run your plunge up against the edge of the platen. The flexible belt will not push away from the plunge. Instead it will curl up into the plunge and sand it. Use a little pressure to get all the way into the plunge.
To do be able to get in a plunge with a belt sander can be very tricky. You want to be pretty careful not to sweep your plunge out. Personally, I like my plunges to be nice and crisp so I use a file guide and a big bastard cut file to finish them up. Then to get out the marks just put the guide back on and use sandpaper. Make sure to get out any scratches before heat treat though.