Slicing adds holes/cracks to my object
here is just a addenum to Tom van der Zanden's answer

this is (an example of) what you may design - nice object with virtual outline, and virtual fill
so this is what you see (and what you potentially expect)



but here is what you get (and probably not really expect)



red parts are the areas which are not covered by fill because nozzle cannot reach there

solution is 


to redesign your object in smart way or
to use smaller nozzle or
to use smarter app (like Slic3r)


and of course you can use all 3 options together to get best results ;)

EDIT
here is simple explanation why smarter app could do the thing


and here goes the difference



green parts are new covered areas

not much but somthing extra