Mesh Thicken help

Hello I am trying to thicken the mesh of a car that will be printed at 1/40th scale. I am trying to thicken the inside of the mesh so it has a surface I can print. How can I manipulate the thickness? I have weaverbird and have the mesh thickness on but I do not know how to alter the thickness past the default it changes too. Also the thickness would be sort of on the inside, not the outside so it would maybe be a negative thickness? I am new to grasshopper. Thank you for your help in advanceSaturn_98.3dm (3.8 MB)

The thickness in some parts will give you wrong result

You don’t need thickness, you need shelling. Thickness wont take care of self intersections on parts that are concave internally. Use some software meant for 3d printing which has algorithms for this like Meshlab or Meshmixer (both are free). Additionally, usually the slicer software for the printer will allow you to assign thickness to the print without having to model it.