You need to remove it.......
Try one of these attachments for a grinder..
Cupped Diamond Wheel
Fits standard type 115 mm angle grinders (100 mm x 22.2mm bore.)
Ideal for grinding away bumps in uneven concrete floors and keying old tiles or terrazzo before tiling
click picture to enlarge
weight 275g
Price: £18.88 £21.71 Including VAT at 15%
Wire Brush for Angle Grinder
A very aggressive wire brush for use in an angle grinder - You will be amazed how fast this works!
Use to remove all kinds of unwanted paint, adhesive, rust and other surface contaminants etc
Very good for 'keying' smooth surfaces.
- Hardend & twisted 0.5 mm steel strands
- 75 mm Diameter
- Standard M14 thread (fits all standard angle grinders)
- Max speed 12,500 Rpm
We recommend you wear suitable eye protection overalls and gloves while using this tool
Price: £4.49 £5.16 Including VAT at 15%
Both available from trade tiler.
Then you can prime and tile...some comps say don't mix primers but then an acrylic primer is a primer...