Infill changes affecting the shell

I recently printed a sanding block from a file I downloaded, but I have had the same issue with other files as well. So I printed one with 50% infill set. The shell looked very rough and I would guess it is also at 50%. I then printed one with 100% infill. That part came out perfectly with a smooth shell.

Am I thinking incorrect that the the shell and infill are separate? What do I need to adjust to fix this? I am using Cura for robo 2.5.0. I will get some pictures uploaded later today.

Thanks in advance for any help you can provide!

Brian

 

Sorry guys.. been away for the holiday weekend! OK. so below are the settings from Cura that I really have not messed with. I can only assume that I need to tweak them. Suggestions?

Wall Thickness: 1mm
Wall Count: 2
Top/Bottom Thickness: 2mm
Top Layer: 20
Bottom Layer: 17

Top and bottom of the part is fine, regardless of infill settings.

Brian