Questions about the Hair Spray technique.

I have a Ma. K Polar Bear kit ( http://www.starshipmodeler.biz/index.cfm?fuseaction=product.display&Product_ID=1181 )and I was wanting to weather the edges of the feet and body.  I have read about the Hair Spray method and I had a few questions.

First make to sure I know what I am talking about.  With the hair spay method you do your base coat of "under" color.  Let Dry then spary the model with hair spray, and let dry again.  Then spray your base color and let dry.  Then take a brush and water and scrub where you want the paint to come off, till it looks like what you want.  Is this correct?

But my questions is, once you have scrubbed off what you want, do you need to seal the model again?  Is the paint safe once it has been sealed?  Also could you use the hair spay method multiple times with different colors?

Thanks,