I am doing a stairway makeover and have removed the carpeting.

The treads and risers are plywood, probably exterior grade. The treads are bull nosed and need wood filler,throughout to make them somewhat presentable.

The balister and spindles are currently stained a golden oak.

I would like to RE-STAIN the balister a deep cherry black and RE-STAIN, not paint, the spindles an off white.

I would like to STAIN the plywood treads And risers the same color as the spindles.

My question is has anyone re- stained going from dark to light and how would you treat very rough plywood treads with a stain. Many thanks.

First, stains only stain bare untreated wood. They do not work on already stained or finished wood. There are 'stains' called gel stains. Those are paint, not stain. They are, however, your least expensive option for darkening the balusters.

You cannot stain---gel or otherwise---lighter.

The other staining gotcha is that different types of woods stain different shades when the same stain is applied. Meaning nothing will be the same color.

I've never seen bull nosed plywood treads. Treads are usually at least 1" thick, and more often 1&1/8" thick if made of hardwood or 1&1/2" thick if made of construction lumber. Construction lumber treads are designed to be covered by carpet. There is a good chance someone will get splinters from construction lumber threads at some point since the wood is way too soft for staircase wear.

You could sand all the finish and most of the stain off the balusters and spindles---that would take more money in sandpaper than it would cost to buy new spindles.

End advice is to get new materials.