by Phil Edwards
(Watford)
Hello,
I have removed ceramic tiles from a shower enclosure and have exposed a sand/cement render that is mostly sound (there are some areas that will need to be made good) and approximately 50% covered with the remains of old tile adhesive. In order to make the wall perfectly flat and plumb in preparation for new ceramic tiles I intend to plaster it. Plaster depth required varies from approximately 1 to 5mm. My questions are 1) how do I ensure a good bond between new plaster and old render that will take the weight of the new ceramic tiles? and 2) which plaster should I use?
Looking forward to hearing from you
Phil
Answer
Hi Phil,
1.
If the surface you are plastering is rough rather than smooth, of varying depths and ridged then you already have a good key. However to be on the safe side a coat of diluted PVA (follow instructions but a milky consistency is good) will ensure a good bond. New plaster should be applied when the PVA is tacky.
2.
Sand & cement mixed in the ratio of 4:1 or 5:1 (with a drop of plasticiser) may work best. The gypsum based Bonding Coat will live up to it's name and provide great bonding qualities - but, if there is any water ingress due to any defective tiling/cracking etc. then Bonding Coat will absorb as much water as it can - creeping up the wall and eventually turn to a Mush. Whereas sand & cement will remain unaffected.
Paul