Putting fresh thinset directly over an existing layer of old thinset is generally discouraged unless certain strict surface preparation conditions are met to ensure proper structural bonding. If the old thinset is securely bonded, structurally sound, completely flat, and thoroughly cleaned of all dust, grease, and loose debris, you can sometimes apply a modified thinset over it, provided you first apply a quality primer. However, if the old thinset is crumbling, uneven, thick, or contaminated with old mastic adhesive, it must be completely scraped, ground, or scarified away. Installing new tile over a compromised or thick buildup of old mortar severely risks bond failure, leading to loose tiles and cracked grout lines down the road.
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