I'll watch the video later today.
I'm not mechanical, but any drawings I've done up till now have been 2D. So I'm use to say wanting a box 1" away from an edge, so I'll draw a 1" line, draw one side of the box, erase the 1" line, and continue drawing the box knowing one side is exactly 1" away. With 2D (such as AutoCAD), what you see is what you get.
The feeling I'm getting with SketchUp (not that I'm defending the need to use it over other software) is that it's extremely basic and not smart. The cover that I was having issues with is for a very basic box I designed using the same methods (laid out a rectangle, gave it some thickness with push/pull, scaled the outer edge of the rectangle, and gave that height using push/pull). I built a circuit and was afraid the exposed leads would short, so I created a rectangular box as a way to practice.
The recent project (the cover to cover the box) was a way to practice a bit more and have a cut out on top for DIP switches. The assumption I have is the mesh issues were a result of incorporating my 2D experiences into 3D work. I'm guessing SketchUp saw some extra lines I used to get the cutout in a particular location and I forgot to delete them (or was unaware they were reinserted after performing an 'undo'), and it created triangles.
Using the automatic repair in the software is nice, but, if I'm making errors, I'd like to avoid them instead of having the computer fix them for me (it's similar to saying I don't need to learn English because auto correct will fix it for me). I think at this point SketchUp is too basic and I'm getting frustrated over issues that are not necessarily due to my inabilities, but because SketchUp needs to be pampered.
As stated, the cover scale wasn't correct (I appreciate the time everyone put into fixing it), and I tried sizing it down by 25.4 (thinking it was in inches and needed to convert it to mm, however, that didn't work. Ironically, I took a guess at reducing it to 10%, held it side-by-side in the slicing software to the box I made, estimated the comparison looked like the correct size, printed it last night, and it was exactly the correct fit. I don't know why 10% worked, however, as stated above, it's a matter of pampering SketchUp and think it's a waste of mental resources to worry too much about it.
In any case, one thing I learned from this recent thread: the slicing software interprets dimensions as mm, so now I use those units of measure instead of inches.
Even though I'm on the electronics side, I'll admit, mechanical drawings are fun.