Glen,,,, people use correctly measured amounts of waterproofer in scratch coats all the time with no adverse problems. You say you had to take off the previous s/coat because it was powdery, and you're blaming the waterproofer. Because of this, you have scratched the wall again "without" using waterproofer, but you now have a problem with high suction.
A proper measured amount of w/proofer would not cause mortar to go powdery. "Too much" w/p in the mix would, as would drying out too quick, or too much sand in the mix. You now have a problem to solve with the suction. As Joe suggests, giving the wall a really good hosing down 2 or 3 times will help. I've not tried the SBR method,, but to blame the failure of the scratch-coat on your interpretation of a measured amount of waterproofer i find hard to believe. Did you use a cement mixer or did you hand batch. Was the sand and cement correctly guaged? How did you measure the w/p and what brand? Was the weather warm/hot, windy when you scratched? Was the brick/blockwork dry and absorbent, or did you wet/damp down the wall first? Sorry for all the questions, but I think there's more to the failure of the scratch coat than the w/p.