Circles "heal" over on exported .stl files


very often (VERY) circles on my models will “heal” over (develop a surface) in the .stl exported versions of the model. i can look at the .stl model in several different mesh tools (meshlab, netfab, etc) and sometimes there is a surface on the circle and sometimes there is not. concentric circles cause more issues than isolated circles. when i print the model using a commercial printer like shapeways, my prints come back without the holes in them…?? sometimes the model shown by shapeways before printing looks fine and when i get the piece the hole has healed? my partner in crime (modeling) has the same issues. it’s driving us nuts. does anyone know what’s causing this or how to make it STOP…?


It’s very hard to just guess.
If you upload a model that exhibits this behavior we can take a look and maybe spot the problem.


Are your models watertight (solids)?