Yes you could do it with a sponge, for example if you wanted a light to dark pink I would do 2 layers of cake pop curing each, then using a sponge I'd dab on pink bikini to create your ombre look and cure. Add more if wanted a cure and maybe some cake pop too making sure to do a full cure and that its not too thick when curing. Also making sure you clean any Shellac that may have got on to the skin before curing.
With additives its so much easier. Cure your shellac colour layers then take a small amount if additive and blend it down the nail. A lot quicker and less messy.
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