These are helpful tips on creating 3D printable objects in Google Sketchup. However, I still myself sending my exported STL to http://cloud.netfabb.com to fix all the mesh issues. Are there better ways?
http://www.mastersketchup.com/8-tips-for-3d-printing-with-sketchup/
TinkerCad - too bad they’re shutting down… 
Tinkercad is alive and well and under new management (Autodesk). And I’m glad to say that the platform is far more stable now. I can actually count on saves as having occurred. The only weird thing is that at some point I get a warning that I have only some small number of edits left. Not sure if it is per project or the fact that I have a free account and I’m reaching the limit of projects stored on their servers.
Hah? They’re not? News to me… I can’t find a single indication they aren’t, or of an AutoDesk takeover. Link?
Never mind - found it! Yay! 
The actually did shutdown, then reopened a few weeks after.
These Sketchup tips helped me. I’ve been having a devil of a time with non-manifold stl files. I applied a couple of the tips and it solved the problem for the latest thing I was trying to design.
Don’t use Sketchup if you want manifold STLs. Aside from Tinkercad, Autodesk Fusion 360 is free till the end of the year, after which the licensing is pretty decent given the software’s capability, Only revelant if you’re using Windows 7+ or OSX10+
Honestly, Sketchup’s ease of use doesn’t make up for the trouble it causes with attempting to create manifold files. Some people can’t even get the STL plugin to work.
Tinkercad may be easy, but it’s a cloud app… I do not trust those (I like my software and my data on my own PC thank you). What happens if Autodesk decides to pull the plug like the original company did?
I’m all for open source solutions, even if their learning curve is usually steeper.
What I typically try to do on Tinkerad is create the basic part and then download my results. I then import the file into FormZ’s Bonzai 3D. I haven’t got my workflow sorted out yet, but once I’ve saved my new, more precise STL’s, I have tried to use nettfab on them. I’m not to the point of printing yet, at least not the parts that I want the most. Many of my novice designer parts are public, CC, attribute or share-alike. I’m not too worried if they get copied or shared, printed or given away. However, not all parts are created equal, so some parts are born on the local machine, for whatever reason deemed appropriate at the time. Some models have been uploaded to thingiverse, shapeways, imaterialize, flickr and a few others.
@Normand_Chamberland - what problems have you experienced with the STL plugin?
@Thomas_Thomassen , the symptom is Repetier shows mesh error. The problem most likely is in the model and not on STL plugin. However, it would be awesome if STL plugin would do whatever netfabb does to fix it.
I love sketchup for the models they have. I wish there was a way to"solidify" the models that I’m exporting. Right now, I’m tracing and combining.
@Hari_Wiguna have you tried other slicers, like Kisslicer, for example?
@Patrick_Ryan No I have not.
@Hari_Wiguna Add any bugs, issues and feature requests to the Issue Tracker at GitHub. That way we can keep track of what goes on and what we might try to add next.
@Chris_Reyes Are you talking about arbitrary models which wasn’t initially made to be models? Or is it models that’s intended to be solids but with minor errors?
@Hari_Wiguna You can use external slicers to generate gcode that you then load in Reptier. You might try that, to see if your models can be successfully processed by another slicer. I’ve just started to use Kisslicer, and think it might be giving me better results than Repetier’s built-in Silc3r implementation.
@Thomas_Thomassen arbitrary models. Similar to the way I can trace a bmp and make it an avg. Their buildings are awesome.