This implies that the honesty is a novelty. That in prior conversations – or the entirety of the conversation before the honesty disclaimer – the speaker was not being honest. This is a horrible way to build a connection with someone. It is a poor way to establish trust. It is an impossible way to build a tribe.
Instead, speak honestly from the outset. Leave no question about your honesty, candor, and credibility. If you do, you will never have a need to let someone know that you are (now) being honest.
“Honesty is not a courtesy, anyway. It’s a must.”