[Histonet] How far will stain reach into a thick section on a slide?

[CM Bush]

Dear Histonet,
 
I am wondering what you can tell me.  We are staining thick sections of human brain.  The brain has been very well fixed in 10% NBF, and sections are mounted on slides.  We are using thick sections (50um) and Nissl or immuno staining.  Does any one know how deeply Nissl stain (or an IHC stain) can permeate tissue mounted on a slide?  Is, say, any tissue thickness greater than 20um (for example) moot as it will never be reached by the stain? 
Would staining floating sections be a more rational way to deal with thick sections? 
 
Thank you vey much for the help.
Clarissa