Advice on a materials,or technique.

Hey everyone,I know that results vary from person to person and ways of achieving certain results are a matter of personal experience and personal preference. Here's my situation. I have a picture that I need to get 2 medium sized areas very dark. when I begin to darken an area I get a "grainy" look to the graphite. I've tried several different hardnesses of pencil,different shading practices including layering the graphite and varying the amount of pressure I use. I am using Strathmore 400 series paper. What practice,paper or something have you used to avoid this. I know alot of people will probably say try different things and use what you like best,I agree with that completely I just need someone to point me in a direction.....Thanks so much for any advice.