Bad object surface above supports
Generally speaking, it’s always the best option to rotate the object in such a way that the supports are not needed at all. However, if you need to print complex objects, you will end up using supports anyway. After you remove the support structure, you may notice that the surface above the supports looks worse than the rest of the print. As you can see in the photo above, several strings of filament are actually coming off.
How to fix bad looking surface above supports:
Try to rotate the object in Slic3r PE and find a better orientation that would require fewer supports.
Decrease the gap between supports and the object – in order to make the supports easier to break off from the printed object, a small gap is left between the support and the object’s surface. If you decrease the gap, you may get better results, but the supports may be more difficult to remove. You can tweak the values for supports in Slic3r PE, section Print Settings – Support material. Try fine-tuning Contact Z and Decrease pattern spacing to improve the quality of the surface.
Tweak bridging settings – layer printed above a support structure is essentially a bridge, so tweaking the bridging parameters can help. The very basic thing you can try is increasing the cooling (fan speeds), slightly lowering printing temperatures or slowing down the print speed. See ‘Poor bridging’ article to learn more about it.