How to remove lots of small geometry without affecting the rest of the model?

I quite often open and edit STLs… Some of them made of almost contless polygons…

I use tools like Cleanup to reduce some of the geometry, but often the model is still very complex.

I then might push a cylinderer throught and intersect it to make a hole… And then have to delete all of the unwanted geometry and this takes time.

I generally use the select tool dragged from Right to Left, or Left to Right depending on what I want… But this tends to highlight stuff BEHIND what I am trying to delete.

I realised last night that I can use a section cut to ‘mask’ of the areas I want to protect and this has helped… But I just wonder if there are any other tools which might help here…

When deleting lots of complex geometry, is it just a case of going carefully and slowly and keep checking the model for damage? Or are there any other techniques I might have overlooked?

There is no “silver bullet” that I know of for removing coplanar lines often associated with importing other file formats. I often have to clean up STEP files and employ many of the same tricks you mentioned already especially CleanUp. There are some selection tools extensions that can be of help, to select only edges. I do not use Skimp but have heard good things about the service if you do this regularly.

Do you employ dragging the eraser tool? It’s another trick for quickly erasing multiple lines.

Dunno if this will help you any, but there is an extention called “iSelect” by Renderiza. I have an old version (it was compatible with Google SU8), and it comes in handy occasionally. There is now a newer version, 2.0.6, compatible with your version of SU, and it may work even better than the older version did.

The selection window is related/fixed to the screen’s horizontal and vertical directions… You may benefit from using [Ctrl] and orbit (= no gravity) to rotate and viewing your model differently. Thus allowing you to change what portion you select

There are so many options and everyone works differently
It might be easier if you attached a specific .stl that you want to modify with the modifications you would like to make in a skp file so we can show you how we would tackle it.