Outside on an overcast day it definitely can work, unless your painting is very glossy. It would be good to add a grey scale chart to the photo to adjust the white balance accordingly. If you have trees around they might cast a greenish tint. But there is another problem if you want to take multiple photos to merge them because even on a overcast day the light intensity is changing all the time and if the differences are too big it will be difficult to get a consistent scan.