Is the car actually loosing water? There are other causes of white smoke (running rich on a cld day is a common one). As has been said, look for signs of water in the oil and oil in the water. You could also ask a garag to do a compression test (which shouldn't cost much). That might be helpful in confirming the diagnosis.
If the car is really going to be scrapped anyway, I'd just live with it for a while, topping up oil and water as required. A car can run with a minor head gasket leak for quite a while. To answer the OP's original question though, Radweld and other similar products won't help - the pressures in the cylinder are too high. As a student I had a mate whose old Hillman Hunter had a blown head gasket. He'd been told that an egg white could sometimes fix a minor coolant leak so he took a spark plug out and poured the contents of an egg down the hole...
...on tunring the key, there was a thud as the cylinder hydraulically locked and the tiniest ooze of white "poached" egg from between the head and the block!
and that was the end of that!