We want to add a subway tile backsplash in our bathroom but don't want to strip all the paint first since the wall's in decent shape. I've heard different opinions about whether to use thin-set mortar or premixed tile adhesive over painted surfaces. What actually works?
You need to bite the bullet and strip the paint - or at minimum rough it up significantly. Thin-set mortar or premixed adhesive won't bond reliably to a smooth painted surface, no matter what anyone tells you. Paint is a barrier, and tile adhesive needs to grip the wall substrate itself, not a slick coating on top.
Here's what actually works: use a paint scraper or orbital sander to remove the paint down to bare drywall or plaster in the backsplash area. You don't have to strip the entire bathroom - just the 18-24 inches behind where your tiles will go. If the wall underneath is damaged or crumbly, fill those spots with drywall compound or patching plaster and let it dry. Once you've got bare, slightly rough substrate, thin-set mortar is your best bet for a subway tile job. It's stronger than premixed adhesive, won't sag as easily on a vertical surface, and lasts longer.
If stripping paint really isn't an option, your only semi-reliable workaround is to prime the painted area with a bonding primer (something designed for slick surfaces), let it cure fully, then use a quality premixed adhesive rated for vertical applications. But this is honestly a compromise and more likely to fail down the road. Taking an hour to scrape the paint off properly now saves you a failed backsplash later.
The real issue isn't which adhesive you pick - it's that paint is slick and will eventually let tiles slip. Most folks who skip prep end up with loose tiles within a year or two. You need to rough up that painted surface with 120-grit sandpaper or a sanding block, wipe away the dust, then use a bonding primer designed for glossy surfaces before applying your thin-set mortar. Premixed adhesive can work but thin-set gives you better long-term grip and flexibility, especially in a bathroom where moisture and temperature swings happen constantly.
Sand that paint down with 120-grit or rougher sandpaper until it loses its shine - that's honestly the only real fix that won't leave you with tiles falling off in a year. Skip the stripping entirely and just go with aggressive sanding, then use thin-set mortar (not premixed adhesive, which doesn't grip as well). The earlier answers aren't wrong about the paint being a problem, but they're overstating how much work it takes; you don't need to nuke the whole thing, just break the glossy surface so the mortar actually has something to grab onto.
Paint prep is definitely the crux here, and everyone's right that you can't skip it - but there's a pitfall worth highlighting: if your bathroom gets steamy and humid regularly, even roughed-up paint can start peeling away from underneath the tiles after a few years, which means your whole backsplash comes with it. Roughing up with 120-grit or coarser gets you 80% of the way there, but consider whether stripping that paint entirely might actually save you a headache down the line, especially in a moisture-prone space. If you do stay with sanding, seal that wall with a primer designed for slick surfaces before tiling - it's an extra step but prevents the paint from becoming a failure point later on.
The adhesive choice matters way less than everyone's making it sound, but here's what actually trips people up: even roughed-up paint can fail if there's any flexing in the wall behind it, which is super common in bathrooms where studs might not be perfectly backed. You could sand it to death and still have creep if the substrate shifts, so make sure your wall's structurally solid first before picking your adhesive.
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