Baronav8

I Just bought a 93' ES 300 looks and runs great (it has 100k miles). Recently the car started to spill coolant from the overflow tank when the engine is shut-off. (not all the time). When I open the plastic cap on the reservoir tank, the fluid looks hot and appears that bubbles are coming out of the hose that goes into the overflow tank.

My mechanic told me that the cooling systems is contaminated with rust. In fact it is. He flushed the system, but he says that it will not cure the problem, and eventually the radiator may get obstructed. He says there is no long term solution to the problem.

The temperature gauge shows normal operating temperatures during driving and idle.

Can the rust problem be solved?
Is the spill of coolant out of the reservoir related to the rust problem or can it be something else (maybe a bad thermostat)?