I have created a surface, knitted it and I am now trying to thicken it. The part needs to be 2 mm thick. I try to thicken the inside of the part (concave) however i get an error that says "Rebuild Errors- Failed to offset or face could not be deleted". If I go to the outside it works, but does not give me the right shape, and it squishes the outer lip and body to close together. How can I make this work or what alternatives are there to making a surface into a solid? I am new to surfacing. Thank you.