Can't generate face groups accurately enough, Fusion won't select individual face

[alan.jonesXHXUF]

I've been given an STL to reverse engineer and I'm struggling as to generate the face groups I need, I need to be able to select faces with more precision than Fusion is allowing me. For example, here I'm using the create face group tool, have already selected the 2 large faces individually:

But when I try to select some of the other faces that make up the flat base, Fusion also selects bits of the filleted corner or the cylindrical sides of the hole:

Which is absolutely no use as these need to be in separate face groups. What am I doing wrong? I've tried the auto generate option but it always gets it wrong too and I can't refine it. 

[TrippyLighting]

This object looks simple enough to be re-models using the native tools in Fusion 360.

If you can share the mesh file, that would be helpful.

Facegroups can be edited and maybe that's needed here.

[wmhazzard]

Try using the paint selection tool to select the faces that you want. 

[laughingcreek]

is your brush size at the min on your mesh selection pallet?

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@alan.jonesXHXUF wrote:

... I've tried the auto generate option but it always gets it wrong too and I can't refine it. 

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on some meshes I auto generate faces with a setting that breaks things up a little more than needed, then use "combine face groups" to combine separate groups that should be the same group. 

TBH, when working with meshes, about 95% of the time I end up just remaking the part natively in fusion, using the mesh as a visual reference only.