“The stair risers in my home haven’t been painted since construction in 1996. Scuffed, yellowed & highly trafficked; it’s time. Original paint unknown – do I prime first & w/ what?
Then the biggie: waterborne acrylic enamel in semigloss or something else? Which brand for this non-pro working vertically w/ stained & sealed hardwood above & below with a wonderful wife raised in a home her Daddy built with His Own Two Hands!?!”
From your description, particularly the year of the original paint job and the fact that the white paint has yellowed, suggests that it was probably oil based enamel originally. As most paint manufacturers (and the industry as a whole) are moving away from oil based products, you are in the common position of an “oil to latex conversion”.
There are a few steps and some product selection that need to happen, as well as protection basics.
First step is to thoroughly scuff sand the existing white paint on the risers at 150 grit. You can do this either by hand or with a power sander (carefully). Either way, vacuum the dust as you go and wipe with a tack cloth or damp cloth.
Next, you need an intermediate coat to serve as a bridge between the old coat and the new. A primer with strong adhesion qualities is best. One good one is Sherwin Williams Multi-Purpose latex primer. While you are at the paint store, grab a roll of low tack (orange label) blue painters tape and a roll of 6″ craft paper for protection. Use the tape and paper to protect the underlying stair treads and floor sections that you don’t want to spatter paint onto.
Brush the primer on with a 2″ brush. After a few hours of dry time, lightly scuff at 220 grit, clean and you are ready for paint. Wash the brush well when priming is complete and you can use the same one for the paint. When selecting paint there are many options from the major manufacturers. Given that your existing oil paint has probably faded considerably, you could select a satin or semi gloss sheen for a fresh look. Apply two coats with sanding at 320 in between coats.