Hi, I had this model in which I used weldments for the pipes and now I want to 3D print it but the pipe's inner diameter is very small so I wanted to remove the hollowness and needed complete solid pipes instead. Can you help me changing the hollow weldment pipes to complete solid ones?

I have no idea how to do so.

(You will have to zoom in on the model as the model is small / It has been scaled down)