New to Blender, how to make texture paint transparent? I'm completely new to Blender.  I'm using 2.79.  This is my first model.  My question is, when I select my model (not completed yet), and go to 'User Persp", and select 'Texture Paint', I'm able to draw different colors on it.  That part is good, but eventually I'll be adding seats and engine to the model.  How can I make the seats and engine see through from the outside?

I already tried clicking on the 'Transparency' check box, but the model is still 'solid'.  I'm attaching an image of what I have so far.  

This is the link to my model.  Its not complete yet.  https://drive.google.com/open?id=1Alr1C2c8XKotmqUeJwA6Ldbtf_fvK3sL
In the latest version of Blender (version 2.82), you can setup brush, alpha blend, render engine and nodes like this to get your results: