We just had baby wipes with us - but we also waited too long and it was too hard so nothing really stuck to DDs hands. Maybe vaseline? Good Luck - I hope it works for you.
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I put my daughter's hands in plastic baggies before pushing them into the concrete...but only one side came out looking like a handprint because I was afraid I was going to hurt her. I bet vaseline smeared on the hands would work.
We tried handprints and ended up doing foot prints because he was just trying to grab handfuls of the concrete! We just dipped his feet in water right away and it washed off easily.
Re: quick help needed - hand prints in concrete patio