Bob, even if you do kill the weeds now, they will come back eventually. Underlaying with landscape fabric will help. If we are calling "chat" the same thing--small stones--it will take 6 or 8 inches of them to pretty well keep the weeds out. The landscape fabric will help keep the stone on top of the dirt for a while longer but eventually the dirt will start mixing with the stone. Sounds to me like you are going to have to remove a heck of a lot of dirt--which you should be able to sell. The bad news is you are going to have to buy a heck of a lot of chat.