If the issue is that there are lots of surface depressions then you should be able to face-fill them using a wide filling knife or caulking blade. Apply the filler and then drag over the blade a second time to remove excess from surface of wall. Once you have sanded the filler and given it a coat of paint you may notice areas that need further filling. It wouldn't hurt to thin down the paint a bit with perhaps 25% water depending on how porous the filler is. If there are bumps which are proud this is not something that can be disguised with filler - it would need to be sanded or scraped.
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