Sales tax exemption for rehabs?

[Uwe K.]

I am just coming from a friend's company where a sales rep and office lady discussed if certain items should be sales tax exempt because they are being used in the manufacturing.

This gave me the idea that the sheet of drywall from Homedepot that's being screwed on the rental housing wall could be sales tax exempt?
I am probably not the first thinking this, but any merit to this thought?
It technically goes into the end product. Although the end product is not really the house, it is the right to live in there.

What of flipping?

Never read much on sales tax (this is in WI, if it matters). Or does this expemt stuff only apply if you charge customers sales tax?