Caulk…but usually done before painting…you will need to touch up.

Thanks Susan, I figured as much. Should I just use a white caulk? The paint color I used for the risers is Benjamin Moore’s White Dove.

I would use painters tape (prefer frog tape to others) and place the tape on the stairs, then put a thin bead of white, paintable caulk and smooth, remove tape.

To get perfectly straight lines I use frog tape and seal the edge with the underlying color prior to painting with the wall color. In this case, with the stairs being wood I would use a clear latex paint. The only one I have is Rustoleum Painters Touch clear. Not sure who else may sell a clear latex paint.

https://www.amazon.com/Rust-Oleum-242057-Painters-Touch-Quart/dp/B006ZTHBJ8/ref=asc_df_B006ZTHBJ8/?tag=hyprod-20&linkCode=df0&hvadid=167138874211&hvpos=&hvnetw=g&hvrand=10194335253973676554&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9006612&hvtargid=pla-309606691172&th=1

here are instructions on how to stop bleeding under your tape.

https://nadinestay.com/blog/how-to-paint-line-without-bleeding

No the gap wasn’t always there. The stairs were covered in carpet. We ripped out the carpet and had to have custom made treads and risers) made out of red oak). They were installed as two separate pieces so that is why there is a slight gap. Now that the risers are white and the treads red oak color, you can notice the gap. I’m wondering if caulking the gaps would be the solution.

I would caulk white like the stringers you just painted. Make sure you tape off the steps, and the stringer, right above the gap. Carefully caulk it and smooth it out, and remove the tape right away. This will keep the caulk from smearing on your fresh paint and also on the steps. Take your time, and do it carefully.

No tape please. Cut a small bit of tip off your paintable white caulk. Push the caulk into the gap without overfilling too much. Spritz the caulk with Windex, spit on your finger, and drag it down the bead. Wipe off the Windex overspray without touching the fresh bead.

I'd have the steps done in the time you taped 3 of them.

You might want to use a colored caulk that would closely match the Dove White paint and eliminate the need to repaint the hard-to-paint gaps that would be caulked. Perhaps:

• ColorFast Bright White Caulking Non Sanded 10.3Oz Paintable Latex Caulk (scroll down here to see #381 Bright White which looks close to Dove White on my screen).
• ColorFlex - Colored Caulk (color CF8003 Pearl appears similar to Dove White on my screen). Sold by the manufacturer.

Would suggest using a sealant that is near the color of your risers. A sealant is more flexible than a caulk as it usually has a higher silicone content. The material you use to fill the gap will need to be flexible as there is some flexure in the stair as people are walking on it.