Why are two-part wood bleaches mixed before being applied?

[Stephen Tashi]

TL;DR

The two parts of two-part wood bleaches neutralize each other. Why are they mixed before being applied?

Wood Magazine https://www.woodmagazine.com/materials-guide/finishes/get-the-color-out-with-wood-bleach descibes the chemistry of two-part wood bleaches as follows:

Once mixed, a chemical reaction begins, wherein the hydrogen peroxide, an oxidizer, acts to neutralize the caustic sodium hydroxide. Neutralized, the solution loses its bleaching power, so mix only what you can use immediately.

Since they neutralize each other, why are the two parts mixed before being applied to wood?

 

[Borek]

They don't neutralize each other. If anything, NaOH (or, more precisely, high pH) speeds up the decomposition of the hydrogen peroxide. However, decomposition is actually beneficial here - as it produces highly active oxygen (probably in a form of free radicals), which destroy most of the colored compounds. Plus, high pH probably changes properties of the wood making it easier to penetrate.

 

[jim mcnamara]

High pH damages wood fiber connectivity - lignin. If you leave bleach on too long you get fuzzy surface, same with other products:

https://www.fpl.fs.fed.us/labnotes/?tag=green-building&paged=2

 

[phinds]

High pH damages wood fiber connectivity - lignin. If you leave bleach on too long you get fuzzy surface, same with other products:

https://www.fpl.fs.fed.us/labnotes/?tag=green-building&paged=2

Most any moisture will cause fuzzy surfaces to one degree or another but bleach is much worse than most because not only does it cause expansion of the fibers (thus fuzziness) but also as both you an Borek point out, it weakens the wood.