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

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SUMMARY

The discussion centers on the chemistry behind two-part wood bleaches, specifically the interaction between hydrogen peroxide and sodium hydroxide. When mixed, hydrogen peroxide acts as an oxidizer while sodium hydroxide, a strong base, accelerates the decomposition of hydrogen peroxide, producing active oxygen that effectively bleaches wood. This high pH environment enhances the penetration of the bleach into the wood fibers, although prolonged exposure can lead to fuzzy surfaces due to damage to lignin connectivity. Immediate use of the mixed solution is crucial to maintain its bleaching efficacy.

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Woodworkers, furniture restorers, and anyone involved in wood finishing and treatment processes will benefit from this discussion, particularly those looking to optimize bleaching techniques and understand the underlying chemistry.

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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?
 
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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.
 
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jim mcnamara said:
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.
 

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