Ahh the piece I did it on was just a fun little thing for myself, where I used a pencil brush to pretend I was colouring with coloured pencils and it was terrifying because to make it blend properly I did have to do all the colouring on one layer and I had to figure out how to fix a few things in a hurry XD
Doing it for a client piece sounds dangerous :D
to do it in an easy way you might want to turn on opacity on pressure on the brush and increase the smoothing to around 20% if it's on photoshop
XD for client work I'm like a layer freak, every stroke is on another layer