Why you should use a stainer strain
The first step to a perfect, dust-free finish using previously opened cans of varnish or polyurethane is to filter the liquid. The finish in a can you opened last week or last year is bound to have a little debris floating in it.
After you pry open that crusty lid, be sure to stir the contents thoroughly. It’s important to get all of the solids off the bottom and dissolved completely for the finish to perform properly.
To filter out impurities, pour the finish through a disposable paint strainer —suspended in a strainer holder—and into a clean glass jar. Salsa jars are great for this because the brush fits in the opening and the lid seals much better for storage than the lid on the original can. Pick up strainers and a holder at a home center or online at mcfeelys.com. Check out more tips on staining here.