30 year old deck

the deck is very old  maybe 30 years, even.  Pretreated lumber…person before me never did anything to the deck.  It has held up surprisingly well, considering and is rather sturdy.   Some boards could be replaced—flaking and all but still strong and supportive.   I’m not in that position to focus on.  As a matter of fact with everything going on in the house wouldn’t have done anything if a well-meaning neighbor (who, unfortunately, didn’t know what he was talking about) didn’t prompt what I thought would be an easy project.  (I figured his lack of knowing when he started filling cracks with sawdust and glue and then someone else told me that a stain would never hold over those spots…other info soon followed. And it’s gotten quite convoluted.)

Interestingly bought Olympic deck cleaner—awful, bleachy and didn’t do a great job.   Since I didn’t have a power washer I used a do it yourself cleaner:   borax, oxyclean and dish detergent.   Pretty good! Much of the mold mildew and dirt came up. But lots of scrubbing.

I’d like to just protect the deck for some more years.  Don’t even care how it looks as much.   I’m just overwhelmed by all the products and reviews…afraid of putting something down that peels—or that I would have to do tons of prep work to reapply ….would love something protective that is easy to reapply.   Here are the answers to your questions.  Any help would be so appreciated!

• In which state is your deck located? Ny   long island (humid)
• How much sun/shade does your deck get? Faces north. House blocks strong sun.    but there are about 5 boards that get that strong sun AND snow and have weathered.  Could be replaced but not able to do that
• What type of wood is your deck made of? Old pretreated wood
• Do you have mold or mildew issues? Yes. About a ¼ of the deck is behind the house and covered by a tree.   The cleaner got rid of a lot.  But some remains.  
• Why/how did the previous stain fail? Never used!
• What type and brand (if known) of stain did you use last time? n/a