This building was previously a school building, with cement block walls..I need to know if I need backer board, or do I just need to fill in the low spots to make the wall flat and level? Also, what kind of thinset do I need for clear glass mosaics?
whats your intention? just a decorative backsplash and/or accent? as long as the masonry is in sound condition (i.e: joints are solid and there isn't any efflorescence, or deterioration of the block face), and take proper steps to clean the substrate thoroughly (remove any oil/paint). if you had the intentions of making a shower enclosure, i would recommend making further preparations. depending on your choice of tile colour (light vs. dark), i would select the co-ordinating thin-set pigment, and possibly a scratch coat if choosing a lighter colour. when applying the thin-set with your trowel, avoid using the notched edge. otherwise, its possible that you may be able to see the notching through the glass tiles.
You don t need a backer board.. As long as the block is solid and no cracks ( they need to be treated specially) you can go right over the block with a few precautions. The block needs to be clean and smooth. No mortar from the joints and stick out. If the walls are unpainted ( and clean) you can go over them with WHITE and white only thin set.There is no such thing as colored or tinted thin set.And glass tiles call for a strong latex modified thin set If the walls are painted ( and clean) you can use a mastic or the special thin set. Fill in the mortar joints with the adhesive you use. Mastic cannot be used if this is a wet area or going to be a wet area. Any questions you can e mail me through my avatar and check my qualifications there. GL
