Ground black pepper carefully poured into your radiator is a great 'stop-leak'. The pepper find it's way to the hole, and swells up some....much like wood. Trust me, this works. You'll also end up with a slightly piquant aroma in your engine bay.
The pepper easily flushes out again after repairs are completed.
There is at least one truck fleet over here where aLL drivers carry a large bag of ground black pepper in every truck. Cheap insurance for older vehicles.
In case anyone else is asking about the soap - mushed up in your hand with a little water, it provides and excellent fuel 'stop-leak', and once again it is easily removed when the job is done. Fuel will not get through - unless of course it's a very high pressure leak. Good enough to fix up fuel tank and carb leaks etc.