I have a new shed that I built about 4 months ago and I used exterior T-111 for the siding. I did not paint the T-111 right away like I should have. I now have the time to prime and paint it as needed but I noticed the sides are darker from the ground up a couple feet, where the water has hit the ground and likely splashed up dirt and water on the T-111. One end of the building is also darkened from the ground to the peak and I believe that is because most of the rains seem to blow in from that direction (North side).
I tried to rub down the darkened areas with soapy water and a towel but I don’t seem to be removing much of anything. If this is dirt it seems to be settled in pretty good. My question is “should I pressure wash or somehow get these dark areas lightened up or can I simply use some type of good primer to go right over the darkened areas?.
You should use a pressure washer to remove the embedded dirt. You don't need to totally strip the siding just get the dirt out. This wouldn't take much time, no more than a hour. After the siding has dried then use a good primer. Make sure to prime all of the siding before painting.