Hi Peter Nichols
The marks on the brass appear to be extrusion marks, they vary quite a lot in thickness. If you are trying to attain a fine grained finish you have to even the grain to a fairly consistent height, a levelling process.
If I was doing it I would use wet and dry sandpaper used wet with water to which a little detergent has been added. keep the water in a small bowl, just dip the sheet in to keep it wet. The water does several things it washes away the chips stopping them from clogging the paper, it gives a very nice finish and it makes sanding effort a lot easier.
When refining a surface it always saves time to use various grades of any abrasive working your way down grade by grade to the required finish.
"Wet and dry" comes in many grades Maybe start with 320 grade and see if you like the finish. You can go up to 1200 grade which will give a very fine but not polished surface. The grades refer to the size of screen used to sort the abrasive particles the higher the number the finer the abrasive. https://en.wikipedia.org/wiki/Sandpaper
When you buy the sheets of wet and dry also get a cork block, the supplier will have them. I always cut my sheets into 4 quarters you will find they fit nicely around the block. Make sure you do not work across the grain or you will spoil the finish. apply long even strokes. The block will save effort and protect your hands while applying an even force to the paper.
Oh and save all the old pieces of paper just like old men and old wine they mature!. Until the last vestige of abrasive is gone you will find them useful around the workshop. Older sheets will give a finer finish. I keep mine in an open container with a weight on top so they don't curl up when they are drying out.
Regards
John
Edited By John McNamara on 29/06/2015 09:19:06
Edited By John McNamara on 29/06/2015 09:23:30