[Andy]

I can't say for sure, but it's possible that the stuff coming out of Sketchup is not perfectly optimized for use elsewhere.

Leaky means that you have holes or open edges e.g. polygons that are close to each other but their vertices aren't shared. Modeling with a leaky object is fine, unless you want to use Booleans (Booleans need 'solid' objects).

If the object you have looks like it should be non-leaky i.e. no obvious holes, you might try this:

Object->Optmize-vertices
Object->Optimize-surfaces

Then try Tools->Select-leaky-surfaces. If it says you have holes, you could try Object->fill-holes.

You can quickly check for normals being oriented by selecting everything and setting it to single sided ("1S" button). If you look around and see any holes, some surfaces are facing the wrong way. Surface->Unify-normals can fix this.

It all depends what the initial model is like.