Depends on how big a leak, or what you are trying to fix, LOL. The best stuff requires you to drain the radiator,fill with water/stop leak, run for prescribed time period, drain, flush, refill with anti freeze/water.
Bars leak works often if it is something that a stop leak will fix.
Mechanics on the whole hate the stuff as you find it in the water cavity of the block when you knock out freeze out plugs and some say it will plug heater cores and radiators though I haven't seen it. Laughing just thinking of some of the engines I've dumped multiple big truck quanity stop leak in to try and nurse along. Sometimes it will surprise you.
