They say that beauty is only skin deep and this also applies to bleaching as well so do all of your sanding before bleaching. After the wood has been bleached and dried you can do a little very light sanding to remove raised grain, but if you do any more sanding than that you risk exposing unbleached wood.
Bleaching box elder produces dramatic results. The tan areas become nearly snow white and the red is much more vivid, going from dull orangish-red or brownish-pink to a bright magenta-red. How can it be that bleach actually brightens the red you might ask?
First, the simple answer ... let's think of bleaching box elder as the inverse of dyeing wood where subtractive coloration determines the final result. If we applied a magenta colored dye to wood that is tan or light brown, the result will be a duller orangish-red. Bleaching removes the color of the wood, but not the red stain.
The slightly more complex answer involves wood chemistry and color science. Wood consists mainly of lignocellulose which is essentially colorless and a variety of organic extractives that reside mainly in the lumens. Some of these extractives are responsible for the colors in wood. The two-part bleach reacts with these organic extractives to remove much of the color. The bleach doesn't react with the red stain so the end result of bleaching box elder is that we are seeing the true color of the red when it is no longer the mixture of red and the wood extractives.
There's more than one way to skin a cat . . . . uh, use two-part bleach. Some people mix the two chemicals together before applying them to the wood. I prefer applying the sodium hydroxide to the wood and then applying the hydrogen peroxide. I feel like this produces a much more vigorous reaction with a better end result. The downside is that it is messier. The messy part doesn't bother me because I do my bleaching in my garden. I also rinse the wood with a garden hose so that there aren't any chemical residues remaining on the surface. I then dry the wood with a towel and let it finish drying in the sun.
Some people have expressed concerns about wood movement problems arising from getting the wood wet, however, we're talking about the wood being wet for only a few minutes so the wetness is essentially just superficial. Moisture penetration on end grain of box elder goes a little deeper, but what gets wet fast also dries fast. The biggest issue is a little grain raising. If you address grain raising before bleaching it will minimize raised grain during bleaching.