Tile adhesive is a strong substance composed of water and mortar that is designed to hold tiles onto a vertical or horizontal surface. Tile adhesive is designed to last for many years and thus can be difficult to remove. Removing tile adhesive from an acrylic tub surface can be even more challenging since the acrylic can be damaged by sharp objects. The best way to remove tile adhesive from acrylic is to use a solvent to soften the adhesive.
Put on gloves and eye protection to protect from possible splashes from the solvent.
Apply a thin layer of denatured alcohol to the tile adhesive using a rag. Do not allow the denatured alcohol to sit on other areas of the acrylic tub. Wipe up any excess alcohol with a cloth.
Allow the denatured alcohol to sit on the adhesive to soften it. Test the tile adhesive every few minutes by touching it with the putty knife. When the adhesive is flexible enough that you can insert the putty knife under it, it is ready for removal.
Place the putty knife at the edge of the tile adhesive and gently push it under the adhesive to remove it. Scrape off as much of the tile adhesive as possible without scratching the acrylic tub.
Apply another thin layer of denatured alcohol to any remaining adhesive and wait an additional five minutes. Use a scrubber sponge to gently clean the adhesive stain using circular motions.
You can also substitute white spirit for the denatured alcohol. Both white spirit and denatured alcohol are available in the cleaning section of home improvement stores.