I don't pick the s*p*r*lex up until I've finished with the mt ie flat wall,any misses filled and tidy corners,only then do I use it for the final two trowels.i do have consistent results doing it this way and I have made sure I have a good look at the finish when painted and looks good,when I first started using it I did have a few probs when it was painted I could see slight shadows when a light was shining down the side but persevered and changed the way I used it slightly and now I'm confident that this problem does not occur any more.
the problem is that because it is less stressful on the body to use people are being lazy and using it to early and when they should still be using an mt.using it at the correct time on the plaster Is another important factor,we sometimes forget we learned with and have been using an mt,tyzack etc for 10,20 or 30 years and many just think it's another trowel but we are having to teach ourselves to use these flexible trowels in the correct way.