DIY Advice Seeker - late 1970 seventies knockdown texture
With the wall being painted, it will be hard to sand with much effect. The paper will tend to clog up. You can give it a try, though. Rent a Porter Cable drywall sander/vac system from HD or a rental place. Start with heavy grit paper and see how it does. A couple of skim coats and you should be good to paint. Hard to say what it would cost for a pro to do the work, prices vary widely in different areas.....
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