I run into this issue a lot and short of remodeling the whole part I haven't found a great way to fix it.
A lot of times when I import a step file it will have a corner that is designed corner-to-corner for welding or some other reason such as below:
When the file is imported the corner surfaces end up overlapping. Import diagnostics seems to really struggle with fixing issues like this one and I'm not great with the surfacing tools to fix this issue quickly.
Are there specific surfacing tools that are better for this kind of application?