So ive got my X1/9 sketches (located in the Design Technique forum) scanned into photoshop and I want to color the background in just a few simple strokes rather than going around every single X1/9 on that page.
This is hard to explain: lets say your working on paper with an old fashioned airbrush and you want to brush ONLY the background or ONLY one side of the windshield. I know that to do this, you cover up the parts of the paper/sketch that you do not want to have airbrushed IN TAP, airbrush OVER THE TAP, then take off the tap, and you have a nice render.
How do you "cover up" sketches in PHOTOSHOP so that I can easily airbrush the whole page with size 300 and only brush the background and not the sketches?

try creating a simple multiply layer. This will allow you to make your broad backgound strokes, but you will be still able to see your your original line art.

Then select each sketch using a lasoo,path or polygol selection tool. You can now simply delete that part of the back drop....

If this sound too complex for you -I suggest getting 'Photoshop for dummies' so you can learn the basics.

Polo mint is spot on with his answer. The way I do this is to do the airbrushing you are talking about on a separate layer. Then you can either use the paths tool to select the areas you wish to remove the overspray from....or if its a sketch i tend to just erase this by hand (it can be much faster). You may want to check out the site below as this is covered in many of the tutorials their.