I wouldn't use this mate, the better stuff is the wynns stop leak or K-Seal. I work for dingbro and we sell alot of K-Seal to nearly all garages in Scotland and also the feedback from the mechanics tell us that the forte stuff isn't that good.
 
me got me some because he a mechanic so got it cheap, a was going to use the wynns at first. iv put it in so lets hope it helps. it says to put the whole bottle (500ml) in all at once but iv put 3/4 of it in but the level is now a few bit over max, will it be alrite?
 
I personally would never use such stuff. the cost of a new heater matrix and all the other stiff it blocks up is far more than what a new rad costs. I've had a customers car in that he had put k-seal in, the head coolant galleries were a mess :(
 
As dan1720v said above it's not the best thing for a radiator ha. I would only be using products like these as a final resort. Some of the old units i get back in work are unfit for remanufacturing due to k-seal and others blocking and damaging the radiator and pipes.