How can i improve the surface

Hi how can i improve the surface of my print, is it possible to have a smooth top and bottom layer without gaps in lines.

I have attached a image of a raspberry pi case i have printed, the top of the case was face down, so infact it was printed upside down.[url=http://]pi case[/url]

So is this the face that was on the bed?
Could be under extrusion but did you specify solid top and bottom layers?

It looks like it's under extruded (gaps between the lines) and/or you printed on a raft. Have you calibrated the extruder? If not, that's the first thing to do. If you printed on a raft you can improve the surface by reducing the space between the print and the raft, but if you print too close you'll have trouble separating the two. Printing directly on the bed will usually provide a smoother finish.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]