I don't use eggwash often- roughly once a week I make a speedy en-croute or pie with some shop-bought puff. Whatever I make, as it tends to be just for two, I rarely use more than half an egg's worth, and it feels a bit wasteful throwing so much away (for this reason I tend to use milk).
If I use a fresh egg (with at least a week to its expiry date), can I keep a cup of eggwash under cling-film for a week? If the expiry is further away, will that make a difference?