This problem has been bugging me for a while now and I have been working my way around it by making up an acetone and abs goo mix and filling in the gaps and sanding it down but it is becoming impractical to continue doing this.
As you should be able to see from the cylinder object there is splitting in the sides like the layers are separating, this only happens on tall objects that I print.
The biggest issue I have and the one I really really want to fix is the gaps in the tops of some prints, I have no idea what is causing it and have fiddled with the settings a lot in an attempt to fix it, sometimes adjusting the models dimensions helps. I do quite a lot of prints that are similar to the two shapes you see there and sometimes I get gaps and sometimes not. All the 3d models are single solid meshes. Print material is ABS.
To clarify I print a lot of different shapes on top of a little block as shown in the pics, sometimes I get gaps as shown.