Be careful pressure washing. If you are too close or if the pressure is too high, you may damage the product. If you are pressure washing the siding or overhangs, have someone inside while this is being done. We have heard of water coming in where the roof decking meets the wall at the eaves or rake.
We have had other owners and builders tell us that TSP and bleach mixed with water works. We recommend calling the manufacturer of what you plan on using for a stain for prepping recommendations.
We have owners and builders who swear by Sikkens. Below is from an Olympic representative on interior stain, but it generally applies to exterior stains as well. Both Sikkens and Olympic are owned by PPG Coatings:
The stain composition has changed over the years and along with natural weathering and age, there isn’t going to be any way to guarantee an exact match. The formulations/blending that Deck House did again are unique to your homes and something that most retail customers wouldn’t necessarily duplicate due to the intermixing done. You are on the East Coast which has been impacted by the VOC regulations and the alkyd/linseed oil based stains are no longer able to be purchased or sold in MA or MD. The color # 708 and 730 are still able to be tinted, but are no longer package colors. The stains are a “hybrid formula” now, Maximum acrylic/oil, and apply a bit differently and will not look the same as the older oil base stains from the 80s. The best suggestion I make for you to better service your customers is to rematch the newer formulation Maximum to the older linseed oil base stain formulas you created, using your mixing ratios as a starting point. The tint base for the Maximum is product code 79550 and can be used a clear blending agent as well to “soften” the color.
Before washing and sealing the house you may want to take a look at your weather strip around the sash. They most likely need to be replaced. If they are not replaced, they may let water in while washing the house.