Lemons and oranges coated with imazalil
No, you can't wash it off. Part of it is probably that washing methods are not fully effective, another part is that there is diffusion into the fruit, and the diffusion is strongest in the uppermost cell layers. In oranges, this is the peel. 


From Kruve, A., Lamos, A., Kirillova, J., &amp; Herodes, K. (2007, September). Pesticide residues in commercially available oranges and evaluation of potential washing methods. In Proceedings of the Estonian Academy of Sciences. Chemistry (Vol. 56, No. 3, pp. 134-141).

The same paper found 0.64 mg/kg imazalil content in orange peel and 0.04 mg/kg in orange pulp before washing. The regulatory allowed limit is 5 mg/kg. So if your fruit starts out with more pesticide than theirs - and it can legally be sold with 8 times more - the residue will be even higher. 

If you want to follow safe food preparation practices, you have to use organic citrus fruit for zest. Non organic fruit can have pesticide residue from the growing period and still be labelled as "untreated" because it was not treated post-harvest. 

If you are eating only the zest, you can happen to stay under the WHO acceptable daily dose, which is 0.05 mg/kg (human weight, not fruit weight). So if you're a 75 kg man1, you can eat a bit over half a kilogram of orange peel (if it doesn't exceed regulatory limits) and stay under the limit. But 1) you're also taking in the pesticide from the pulp, and while there's less in it, you're eating much more pulp than zest, 2) regulatory limits might be laxer where you live than in the EU, and 3) you're still poisoning yourself, even if it's not enough to become alarmed about it. Why consume one more carcinogen when you can avoid it?

1 it's less for women of childbearing age and children