Havent used premix concrete bags before, but id assume do it much the same as mixing your own stuff. Wouldnt faff about trying to spray anything, personally.

Dont just dump it all in at once. Set it spinning, add some water, then add some of your mix. Let it mix in a bit before adding more mix and water.

I usually keep it fairly wet with the small mixers, only letting it get to your desired consistency near the end. Helps stop it sticking if its wetter while mixing. Make sure you can dry it up enough before full though.

The little mixers are bad for stuff sticking at the back though, no matter what. You can bash the drum with a hammer on the outside to dislodge it. Dont let the mixer owner see you though!