If the ceiling is in otherwise good shape, just fill the texture with wallboard compound. If the ceiling is not a "flat" paint (meaning you can detect a glossy appearance to it), you might want to prime with a good primer sealer first before applying the compound. Bonding agents are for helping new cement to stick to old cement and are ineffective on painted surfaces.
Sand off any high spots in the existing texture, and then spread the wallboard compound over the entire ceiling using wide (8" or wider) wallboard knife. It will probably take a few coats to get the ceiling smooth. You do not have to sand between coats, but you should knock off any high spots or lumps with the wallboard knife (dry) before applying a second or subsequent coat. Sand smooth after the final coat. If there are still imperfections you can spot-fill them, again sanding after the compound dries.
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