Need Staining Advice!

After reading a bunch of info and reviews on this very helpful site and elsewhere, I've come up against some questions and need some advice.

We live near Seattle. Our deck/porch is about 5 years old and made of 2x4's (fir?). Yeah, I know, but it's all we could afford when we refurbished it. It has/had some sort of oil-based stain from the local hardware store that was applied fairly heavily (2+ coats in most places) and has held up pretty well until the last year or two. I recall how incredibly absorbent the wood was last time I stained.

It's in the front of our house, uncovered and gets lots of afternoon sun when it's not raining or snowing.

All of the horizontal surfaces and railings have been sanded down. Everything else has been pressure washed. Basically, we have not removed the old stain on any of the surfaces where it still appears to be in pretty good shape. A good part of that is we're running up against the rainy season, and I doubt we could strip, dry and stain EVERYTHING while the weather is still decent. I can do *some* more stripping if need be, but I'd much rather just clean and add some new stain here and there on those still-stained areas (mainly, the spindles) that have held up well.

I just ordered 3 gallons of Armstrong-Clark Semi-Solid after talking to their rep about the situation, wood condition, etc. I'm not super thrilled with having to use a semi-solid, but we really don't have a lot of people using our porch (save for Halloween), and we need the coverage because the wood isn't great and I don't want to do more stripping.

Has anyone used this product? Any words of wisdom for getting the job done quickly and right? Do you think 3 gallons will be enough to do the job? At this point, I still have a little sanding to do where the wood's super rough, then I'm going to scrub it with oxiclean and let it dry. Hopefully we can start staining next Wednesday or Thursday.

Attached are pics of where we are at this point.

Thanks SO much for reading and any help you can provide!