Hydrogen Peroxide is an unstable chemical which ultimately converts into water and oxygen. It is very reactive which is why it is very useful as a disinfectant and sterilizing agent, and used extensively by orchid growers.
When to use
Hydrogen peroxide is a very cheap and highly effective treatment which can be sprayed on all exposed portions of your orchids. This should be performed the moment you witness any discoloration, sunken spots, areas prone to water accumulation (pockets) or wounds.
On what you should be using hydrogen peroxide:
Leaf rots: Rotting leaves, especially common during periods of extended rain should be removed and the remaining foliar mass sprayed to prevent the spreading of rot.
Rot spots: These potentially larger problems can be stopped in their tracks if sprayed before being allowed to progress too far.
Crowns and new growth: Any depression where water can accumulate can be sprayed occasionally to kill spores or bacteria before they have a chance to cause rot.
Soil drench: Although it is possible to treat this area with an orchid present too, you need to appreciate that both desirable and unwanted microorganisms will be attacked. However you can certainly treat new growing media prior to use.
Suggested application rates
Use as a fine mist spray straight from the container. Do not dilute or the results will be unpredictable.
Repeat use as required. There are no hazardous by-products and it is not possible for fungus to build up resistance.
Hydrogen Peroxide typically has a shelf life of up to 3 years once opened, however if when you spray it on an organic surface and it does not fizz it is essentially water.