I once saw a very cool MG Gelgoog in the hobby store which had a very rough texture on its armor. It looked really cool and realistic and for the longest time I just could not figure out how the effect was achieved. I tried multiple primer laters but that didn't work and sanding with various rough grades of sandpaper didn't work either.
I finally discovered how it was done. I was using the wrong primer. I had been using Mr. Surfacer1000 from Gunze Sangyo but what I really should have used was Mr. Surfacer500! It turns out that the numbers refer to how 'fine' the spray particles are so a lower number results in a much rougher finish.
Be careful though, if you spray too many layers of Mr. Surfacer500, you can lose the details on your models very fast so use sparingly. This was what I used to achieve the effect on my RX-93 below.
credit : http://gundam_base.tripod.com/
