Infill is rough - how to enhance?
From your comments can be read that you print infill at 200&nbsp;mm/s. 

Know that 200&nbsp;mm/s is ridiculously fast (like high travelling speed), close to the limits of printing on certain machines (for an AtMega)! It is hard for the filament to keep up at this speed. A value of 60&nbsp;mm/s would be a good value to start experimenting. Your infill is not rough, it just failed printing. I have printed kilometers of PETG, normal print speeds for my PETG are recommended at 30-50&nbsp;mm/s by my manufacturer; I get good results at 50-60&nbsp;mm/s. On my Ultimaker 3, 70&nbsp;mm/s is also feasible.



Note that the filament you use seems to have rather low printing temperatures (195-225&nbsp;&deg;C) as opposed to the PETG filament (co-polymer) I'm used to. The manufacturer does not specify advised print speeds (other than "high", but what defines "high"?), but this user posted some of his print settings for this material. The overall speed of 60&nbsp;mm/s seems to support lower than 200&nbsp;mm/s print speeds.