Sinew advise

[Traxx]

Roughing the back,is called scoreing the bow.Sizeing is just as you mentioned.I would let it dry.

[ChristopherO]

Traxx is right, let it dry.  Doesn't take long.
Ferret's bow building page, also, recommends sizing when gluing on a handle riser.  It allows the glue to sink into the wood pours first, which enhances adherence.  It is a good idea.
If I remember right you are planning on using TB3 like I do.  To be fair, I have read that hide glue is supposed to be superior in drawing the bow into relfex upon drying than a synthetic.  Whether this is completely the case I don't know but thought I would through that out for your information.  I've tied my bow into reflex while drying with good results.  
To me the trade off in weather resistence is more important.

[Art B]

How did your Osage static turn out with the sinew backing Christopher? Did you pick any additional draw weight? Got any pics? ART B