Thomthoms plugin is helpful here but you should try to learn how to prevent those problems instead of repairing them afterwards.
You can use a sectioncut and move it through your object. You should always see only the shell faces to get a solid volume (simplest definition: every edge touches exactly two faces).
Now that I have ‘fixed’ the object, I can see what was wrong before (using the section cut)… Problem is, I don’t know how to avoid it.
A lot of it comes from the way that i created the ‘base’ (i.e started with a shape… Created a 2mm off set… Stretched the outer wall up to 50mm, and the inner one to 2mm… I can see how this is wrong… But I can’t see how I would avoid it…
You can’t avoid that, because that is how sketchup works. You just have to be aware on how push/pull works and what solid means and then fix it.
I used to use smooth at 180degrees when I had large models. Where a problem is it won’t smooth.