First it evens out the finish from the previous coat.
Sanding stain with steel wool.
Before the stain remover has dried gently scrape the layer of stain and stripper residue from the wood surface with a plastic scraper or putty knife.
Scrape directly into a trash can if possible.
Then add distilled white vinegar to the jar.
Steel wool can be used instead of fine sandpaper for many sanding jobs.
In my experience assuming the finish is dry there isn t a difference other than the coarseness of the sand paper or steel wool.
Wipe all over the piece with 0000 steel wool using a light touch to remove any surface imperfections.
After carefully wiping off the sanding dust or residue with a damp cloth rub the entire finished surface with steel wool and lubricant in the direction of the grain.
Use medium to heavy pressure and try to keep the pressure and the direction of your strokes as consistent as possible.
The steel wool buffing process is fairly straightforward.
Step 9 brush on a third coat of polyurethane this time use it full strength without adding mineral spirits.
The use or not of wax variability of drying and curing times grit progressions should you choose to sand and polish.
Sanding with a steel wool substitute between coats of stain is useful for two reasons.
Remove additional residue especially from difficult areas with a steel wool pad by scrubbing in the direction of the grain.
Wipe off the sanding dust with a tack cloth.
Using steel wool or sand paper allows you to create a uniform surface for the next coat of polyurethane which generally looks nicer more professionally etc.
In addition it adds some tooth to the surface that helps the next coat of stain adhere and absorb better and more uniformly.
There is much info in previous q as here that touch on or directly answer some of your other queries e g.
Stirring stick paint brush scrap wood add steel wool and vinegar to a mason jar the first step in making your diy steel wool and vinegar stain is to add a pad of the 0000 super fine steel wool into a mason jar.
Using steel wood to polish wood might seem out of place and actually harmful to the surface of the wood but it isn t if done properly.