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Yes. No higher than 80 grit for outdoor surfaces. Better penetration. 60 is typical in our practice due to the buffing brushes we have in that grit. This process also helps to lessen some of the un-eveness of coverage by furniture and other seldom moved items. Staining (adding a pigmented sealer) is a preferrence. Also helps to stave of UV greying and degradation depending on the opacity of the stain being applied. Toners-little opacity, semi-transparent stains-more opacity. Semi-solid greater opacity and solids are totally opaque. Clear sealers are in the vein of toners which have little to no opacity for UV protection regardless of what they state on the label. Rod

General practice after applying any product that is on the alkaline side is to apply a neutralizer. While percarbonates become inert after they exhaust themselves, the pH is still alkaline. Stains/sealers are designed to go onto wood that is in its neutral state which is acidic in nature. Wash, rinse, neutralize, rinse. Rod

Yes. Clean it first, wait till dry then apply. First coat on new wood is going to look much lighter than on older wood because the fibers are still very tight and don't absorb as well as when loosened up by aging and lignin/extractives depreciation. When you are to perform subsequent maintenance applications, clean with oxyclean if is it just mold. If algae develops, skip the oxyclean and use bleach and water instead. (oxyclean and bleach don't get along in a mixture) Brush and then rinse thoroughly or use a power washer to speed up the process but remember you aren't trying to remove the color. Some may anyway as there are no significant binders to hold it on the surface and UV degradation will make it easy to remove. Once clean, neutralize and rinse thoroughly and allow to dry. Re-apply A/C and if mildew is a problem in your environment, you can also use a small amount of Japan Drier in Only What you think you can apply. Not the whole container. This will help the product set up faster and give less time for nature to deposit things that promote mildew regrowth. Yes, still use a mildewcide in only the new container you have. Not the used/partial one you may left over from prior applications. Any remaining product (hopefully very little) should be disposed of once a drier has been added. The product loses its properties in the can and when you go to use it later, it will not work as designed. This is why I state to only add it to what you think you will apply. If you need a little more you can still add it to that amount required. Keep what comes in the can in the state it was once opened (except the mildewcide if you add it. That does not hurt the formula). Rod

Since you are using Pt Pine, you can seal it within 2 weeks of installation according to the American Wood Preservers Association, the organization that sets the guidelines for pressure treatment in wood. If you are going to wait that long, your exterior environment will be a dictating factor on what you will need to do. Oxy clean is easy enough for just gray wood and some mildew growth. Neutralizing with an acid is recommended. I am not sure what stores you have in your neck of the woods but look for a product that has either Oxalic or Citric acid and is designated for use on decks. If you are looking for more specifics, try https://www.opwdecks.com/ If your deck environment has high moisture conditions from a pool, hot tub or pond/water feature or sits really close to the ground and has little to no air passage beneath (less than 2' results in pockets of dead air and high moisture retention) then you may have to look into an additional mildewcide to help stave off mold growth during the drying period which with A/C can be anywhere from 2-5 days depending upon how much sun and shade you have. More sun, less dry time. Rod