I would just prime all the treads with the BIN after washing with Dirtex. It's something you walk on so you want it to be durable so BIN is going to stick the best. Tell the clerk at the paint store that you would like the primer tinted towards your finish color...they will know what to do. The reason anyone would want to use a wood filler on the knots is to fill in gaps or if the knot is an actual hole, not to actually seal the knot. I maybe would juse use EasySand for this or some wood filler or MH Ready Patch....anything would work really, as long as it is not 'lightweight'. I like Muralo for floor paints. Valspar and the big boxes may not sell quality paints for projects like this, though I'm sure SW would be fine too.

So get some putty for the knots, sand everything smooth, and the use the BIN over the whole tread?

Can I use my Zinsser 123 primer for the rest (risers, newel, and railings) or should I do the entire thing with BIN? I'm assuming the BIN doesn't make anything necessarily "smoother" so it would be ok?

I forgot to ask about sanding earlier... what grit should I start out with and what should I go down to? I always get confused with sanding grit orders!

I just looked up Dirtex because I wasn't sure what it was. Looks like there's a couple different ones. Do I want the Liquid Dirtex?

Thanks, I appreciate the tips!

Here is the order I would use. First, clean the treads with Dirtex, the powdered version would be fine...just mix according to directions. After the treads are dry, sand them with some 150 grit sandpaper. Sand the posts and railings with 3M fine grit sanding blocks. Spot prime knots with BIN. Fill knots with whatever filler you are using and then sand smooth with 100 grit sandpaper. Prime all raw wood with BIN and also prime post and rails with BIN. The BIN is best to use here because you want to use a primer that will adhere well to previously varnished surfaces. The 123 may work fine or it may not. Sometimes acrylic primers will not hold back stains when painting over previously stained/varnished surfaces. After the primer is dry, expect all primed wood to be rough. The primer raises the grain. You should be able to use sanding blocks from now on to get everything smooth...no more sandpaper. Once smooth, you can dust and apply finish coats.

"You should be able to use sanding blocks from now on to get everything smooth...no more sandpaper"

Do you mean I can use the same 3M fine grit blocks for the rest?

Thanks so much for the tips!

I've been getting a lot of negative feedback from people about doing this since the steps are plywood. I'm a perfectionist and will be sure to get these are right as I can possibly make them. The edges of the treads are rounded and stick out over the riser which I believe will help the finished look. If it's done right, do you think it can look decent?

Yes, once everything is primed, the sanding blocks are all you should need to sand. The treads look like pine to me. Yes, if they are plywood, then they will probably look bad painted and they will not hold paint forever. Fropm a decorative standpoint, I am not sure that painting these is a good move. There is no basement door so this stairwell is essentially part of the house. You are wanting to paint oak, which to some is a no-no and generally speaking painted stair treads are a downgrade from carpet. But, the picture of the new look does look good. Maybe some others can chime in with decorative words of advice.

Thanks for clarifying the sanding blocks.

I thought they were pine also, mainly because of the knots, but lots of people kept saying throwing the word plywood around. Although neither is really desirable for this.

The first photo I posted is the basement door at the bottom. The 2nd two photos were during the process when the house was built. I posted them to show the steps underneath. The basement door will be open most of the time once finished and I had wanted to run the basement flooring (pergo) in that landing butting into the stairs.

The white spindles were actually oak and I ended up painting them. (I didn't think out the stairs real well) IRL, I don't think they look real grainy.

Also, that odd piece of trim along the stairs will come off and the wall will be drywalled right.

I know you can only put so much lip stick on a pig but I think I'll probably take the plunge. If it does look bad I believe a runner could help it out until we could replace the treads and railings/newel.

It won't be for awhile but I'll post an update if I follow through.

here are my "decorative words of advice":

Please do note that the finished sample you supplied shows a view from the bottom of a staircase, looking up.

When you look up at the sample stairs photo you see the nice white risers... your view is going to be from the top, looking down, at dark stair treads.

Are you sure that is what you want? I think you may be darkening an already narrow stair space a bit too much.

Just my 2¢.