Yeah it is, I like to be able to balance everything on it's own. Probably a throw back to working on large projects with under powered computers. I did readdress the question saying just a stereo mix without any final compression and limiting would be also work.
 
People work in different ways.
 

abacab
 
The simple answer to the OP's question is to export your final mix as a stereo wav file to a folder somewhere on your computer.  Then open that file up by dragging it into a new project in Sonar.  The wav should then open as a new stereo track in Sonar, so you are now ready to do your mastering!

 
That answers it.  Thank you!   The other answers, although appreciated, are just a bit over my head.
 
Greg

Sorry Greg. I will try harder to focus on the answer better next time.

Good to hear that worked out for you!
 
There are a few good mastering tutorials on YouTube if you Google them! 
 
I suppose that there are a few that just throw some mastering plugins on their Master Bus while they are mixing and call it a day...

I don't want to sound like I don't appreciate everyone's answers.  It's just that I'm learning and do not understand a lot of things.  So some things don't register in my brain yet.   But I still very much appreciate everyone's comments.
 
Thanks!
Greg

As I said in post #8, you don't HAVE to do the export/import thing - just plunk some mastering fx on your master buss and get mastering
 
The only caveat is stressing your system too much, though any decent modern system should be ok

highlandermak
So here's a silly question. Why can't you master your master bus? Reason I ask is I've used lursen mastering plugin in my master bus and although I don't have as much control have not been displeased either. In theory couldn't you use the master bus and add your eq, l2, l3 etc... to achieve the same results?

You can put a Boss distortion pedal in the FX bin on your Master bus if you'd like .
The reason I choose to keep my mastering plug ins in the track FX bin is because I like a clean master bus for using Analyzing plug ins in Master bus.
 
If you load up your master bus with FX and then put the Metering plug ins last in the chain, I have found a 3db difference (+3db) using Waves Peak and Waves frequency analyzer.
If you put all your plug ins in the track FX bin and only use the master bus for metering I have seen a true 0db measurement.
 
What im trying to say in a nut shell is this... (Only my opinion)
Keep your metering plug ins and FX plug ins separated. They are 2 totally different animals.
One is for coloring, the other is for analyzing.
 
As I said, do as you wish but you might find this to be a good work habit, ahh or maybe not. that's your choice.
Do a test: put all your plug ins in the master bus and use the Waves peak..(or any metering plug in)
last in that chain (you are measuring track output and FX = Master bus: Sum).
Now move the plug ins to the track and leave only the metering plug ins in the Master bus.
Compare your results.
I came up with a 3db difference. hence I changed my work habits a long time ago to keep these 2 tools in 2 separate locations.

When I think of mastering it's a pretty straight forward process. It is the last step where the stereo file is polished to perfection.  top and tails, fade in/out if needed. Overall level and then possibly some EQ and limiting if needed. 
 
I'm against combining mixdown with mastering unless like said above, it's just a rough demo of the song. 
 
So I export the the song sounding as best as possible and I will might put something like the LP multi band in the master buss bin just to check for overs. I find the multi band useful for pointing out where the issue is so I'm sort of using it like a spectrum analyze .   
 
So the stereo wave export is not finished, mastering is the next step. 
I use a wave editor for this step as they are optimized for working with stereo files. 
If Sonar is all you have then fine, but the job is much easier in a wave editor like Wave Lab or Sound Forge. If you plan on doing a lot of mastering you might want to check one out. Wave Lab Elements is only $99. I got Sound Forge ( basic)  free with a Sony Turntable. It's pretty cool for "mastering" old vinyl.
Of note is most of my Sonar plug ins are also available in my other software. 
 
 
 

This video helped me..
 
https://www.youtube.com/watch?v=-fsWWKs9wN8

timidi
The whole concept of "mastering" is way overblown all over the interweb. Especially when referring to 1 song.
Mastering=finishing. Do it however you like.
 
If it's part of a collection, that's maybe a different subject.

"Overblown" is a relative perception in my opinion.
I find that there is some great info. available.

https://www.digido.com/portfolio-item/level-practices-part-2/