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We put down a 1/2" ply subfloor on top of our floor diagonals.

When the drywall guys were done there is basically dried splatter if drywall mud everywhere.

subfloor with splatters of mud

Do I need to scrape this off before installing tile floor. Our floor guy says that he will put a layer of hardiback board on top.

Is scraping enough or do I need to sand the floor smooth?

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Just scraping it flush with the surrounding area of the subfloor will work fine. At most, I would vacuum areas where thinset is to be used under the cement board.This will pick up the dust in the grain of the plywood subfloor, giving room for the thinset to grab better. Make sure the plywood is well fastened, 6" on edges, 8" both ways in the center of the panels. Screws are best. The diagonal subfloor hopefully was secured better first... nails reset, screws added...

In general I would vacuum everything that gets a finish over it to control future dust.

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Vacuum, then get a scraper like this and run it along the floor as if you were scraping out the mud

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The idea is not to scrape off every bit of stuff, since he's going to put cement board on top, +/- 1mm thick mud puddles don't matter. But check if the blade bumps into something, like a bit of mud forming a bump. But the most important is to check there are no screw heads or nails poking out. The mud looks like it has been scraped off already, so this should not take long.

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