What substance is best for cleaning rod blanks of oil, grease, etc before appling the finish?

Assuming it's new and you mean thread finish... I don't use anything but water and /or a lint free cloth when building it up. Use of any chemical or soap has a chance of screwing up the finish. Also if you get ANYTHING into or on to the thread like oil or grease, you'll want to re-wrap it because it's a disaster waiting to happen.

If you somehow got grease or oil on a rod before you finished it... I guess I'd try a very mild and heavily diluted dishwashing liquid... and I would rinse 3-4 extra times to be sure NO residue remained.

Thread finish reacts very strangely to other stuff. There are countless threads on RBO about finish disasters. Knock on wood, I've never had one- but I'm pretty strict with how a blank is treated and stored before it's finished.

Hope that helps! and- welcome to the forums!

Thanks for the information and reply.  I am  new to rod building,  just finished wrapping one and applied the flex coat to the guide wraps with no problem,  when I applied the finish to the butt of the rod I could not get a good even finish,  small spots would show up where the flex coat would not stay in place.   I had wiped the blank down with a damp cloth and allowed it to dry befoe attempting to apply the finish but for some reason I failed to get a good even coat.

I had wiped the blank down with a damp cloth and allowed it to dry befoe attempting to apply the finish but for some reason I failed to get a good even coat.

If you were to ask the question on the RBO site you would get a resounding (NOTHING), the last thing you touch the blank with before finishing is NOTHING at all.

Better luck next time....