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Whenever I skim coat entire walls in a two coat process, after first coat dries I knock down high spots and throw a quick coat of primer before second mud coat. Creates an extra step this way but makes for applying the second coat of mud much easier and quicker due to the fact that mudding over primer rather than the mud itself, it glides over the primer with no drag and gives you more open time to go back over an area without "waking up" the first coat of mud.