Small details are disappearing when I'm uploading OBJ file. Especially all rivets on tank models. why, what should i do
Make sure that they are connected to the body of the model - if they are floating freely, they will be removed as a rivet alone is too small to print. (If they look connected in your software, try exporting to a different format,or use netfabb studio basic to convert the obj to stl in case it is a numeric accuracy problem with either your obj export or shapeways' obj import software)
Perhaps try running your model through the netfabb cloud service, in the hope that it uses different distance criteria than shapeways and is able to create a fused mesh.
Yup, netfab cloud works. But I have terrible internet connection, also after netfabb fixing file became bigger and impossible to change model. SO I have to run model though cloud after each model improvement