"And please do be honest."?

Can you say this?

And please do be honest.

I'm trying to tell somebody that I wish that they are honest with the reply that they are about to write to me.

Is the "do" part redundant?

Yes, they are redundant, but...

Anecdotally, I have used the phrases 'Please do be honest' and 'Do be honest' as a polite way to invite criticism from someone who might have otherwise been reluctant to comment so frankly. For example:

"What did you think about my paper? And please, do be honest, I don't mind if you thought it was bad."

Other phrases that aren't as grammatically redundant that can be used similarly are: 'You can be honest' or, with a frank tone, 'Be honest with me.'