I believe Weed B Gone is a POST-EMERGENT weed killer. Are my chances increasing?

It's safe to apply WBG if you've mowed the new grass three times. If you overseeded 14 days ago, it's unlikely that you've mowed three times.

However, since WBG only affects plants by contact with the leaves, some of the new grass may survive if it had not yet sprouted.

Where are you? Depending on your location, even if the grass survives your attempt to kill it, it may not survive the summer.

I'm in Maryland. It has rained alot since I overseeded. I havent moved my grass even once as it hasn't started growing yet, and the new grass hasn't yet sprouted. Are the odds on my side that my overseeding hasn't been adversely affected by me using WBG?

Additionally, since it rained just an hour after I finished using WBG and its still raining will this lessen the effect of my efforts to rid my lawn of weeds. I'm hoping I can luck out.

Thank you for your help.

I would be a little concerned that you have no grass germination after 14 days. Tall fescue is a pretty quick germinater, 14 days is at the long end of time for it (unless it says differently on the bag of seed). Have you been watering your lawn 2-3 times a day for 5-10 minutes each time to keep the seeds moist? The Weed-B-Gone max is not going to help your new grass, it can only hurt. I'm pretty sure that it works by foliar absorbtion, so if you have no seedlings, it might be ok especially since it rained. The only thing your can really do anyway is wait and seed what happens, or overseed again. I would give it a week to make that decision.

After I made my initial response, I started thinking that WBG may have some pre-emergent qualities and also may stick around for a little while. I may be wrong. Do you still have the container? It may have something on it about how long it lasts, how soon you can use it, etc.