With wcw backpack you can controll the flow with the hozelock flow control tap that it comes with, I have this and I find it very easy to control.
be carefull you dont burn out pump that way, i have a shurflo 5yrs old , still going,
used on its own for 18 mths , now got a van mount , but still use for some jobs , the 4 speeds are good , i use setting 1 for applying tfr to fascia , conservatory roof's, settings 2 and 3 for windows dont use the 4th speed.
daz