Flooring Tile - Retiling shower stall -- Question

I am in the process of retiling our shower stall. I have removed all of the old wall tiles from the cement backerboard but some portions of the backerboard are covered with old mastic that will not come off. My first question is can I install my new tile over the old mastic residue ?
If so do I need to do any additional prep work to these areas ?

Thank you in advance for any help

Dad

How old is this shower? Were the seams tapeed with modified thinset when it was first done? Mastic will soften with hot water exposure, so you should be using thinset to set your tiles and tape joints. Hot water will also soften the mastic to make scraping it off easier. I'm wondering if it's an old mud bed wall or a cement board shower. Sometimes we get penny wise and pound foolish. The cement board to replace the shower surround will be about $50 and take no more than 2 hours to install with coffee or adult beverage breaks. Have I ever mentioned how much I hate to scrape stuff? If it's an old floated mud bed wall, you have some work cut out for you, but as long as you don't have areas fully covered with the mastic, whereby preventing the thisnset from grabbing the mudbed with each tile and not just floating on a bed of mastic, you will be ok. The mudbed walls would be somewhat sandy and some may have been released from the wall as you pulled the tile. The cement board will be smooth.

Thanks for the info ...

Looks like we need to get out the crowbar and some adult beverages.

Dad