Hello madmacs and welcome to the forum.
This depends on the writer software you are using.
You can use Nero 6 , which is fully compatible with most DVDR(W) recorders. You can erase the disc (full or quick) via one of the pull down menu's.
If you try to write a DVD and pop in a DVDRW in the tray , nero should reply with a message , that this disc is written already and come up with a question if you want to erase it or not.
Quick erase takes about one minute.