tfr20 is the fast acting you only need a soak time of 1 minit max ,if you use ubik2000 virosol ect you need 5 minits to let it dwell to work wich it does , some people are now using bleach in there spray guns wich works or a week hypo mix, benzblack wash is also very good on realy badly stained upvc ,but its not cheep and it only has 3 month shelf life !