Front panel assembly installation
How clean does the rear case surface (where the adhesives go) have to be? I used rubbing alcohol and q-tips but the surface is still very tacky.
The cleaner the better. Try using scotch-brite or another metal sanding material. Steel wool may also work. You want the surface to be smooth and clean or else the digitizer will come back up
I would suggest cleaning it until there is no stickiness. In order for the new adhesive to bond correctly with the frame/housing it will need to be completely clean. Once the new digitizer is seated use some heat around the edges to soften the adhesive then clamp it in place until it sets.
Acetone and Un-Du are my go to when removing adhesives. Goo Gone has some residue left behind. Heat will also be your friend. Sounds like you have the q-tip part figured out. Scuffing will help just remember to clean very well with alcohol after you have all the adhesive up. Dont forget to use heat to activate the adhesive when you put the digitizer back on.