yes max is 30 min with the oxalic acid but you do not want it to dry out, and you can skip the oxygen type bleach, if you have a good power washer, it just strips off the graying and dirt from raw old wood, i use a 3500 psi and it works really sweet, then the oxalic acid will restore the wood to a new wood look. be very carefull to use evenly spaced strokes with the pw wand and not to get to close since you can cut right into wood with that much presure. never use house hold bleach on raw wood, but you can use a TSP, water and clorox mixture to etch old sealer before a primer app. then paint, works really nice. best of luck , dave