Gad, I hate writing this...
It's so much easier to 'show and tell'.First let's get straight on terms.Varnish is oil/alklyd paint...without pigment.Poly is polyurethane...a different beast modified in many ways by manufacturers. If you have 'oil-based poly'... you have some kind of hybrid that I am not familiar with. But it sounds suspiciously like some kind of marketing wiz's idea.Application is the same for either of the more traditional finishes...poly is more forgiving (and I believe more durable).The following 'rules' are for a 'perfect'finish (yeah...Santa Claus, Easter bunny and check's in the mail kinda thing)...you can still get a decent 3-foot finish while breaking most of them.Dont mess with the stuff if temps are below 65 degrees or above 80 degrees. Too low...won't flow, too high...fast dry.Never paint from the can. The brush introduces air bubbles into the medium that dry without bursting between uses, might as well sprinkle sand in there.Take a small clean can, punch a couple of holes in the side near the top so that you can fix a wire across the top about one-third of the way from the side. This is a 'striker'...use it to strike excess finish from the brush. Pour a small amount of finish into this can and work from it cleaning thoroughly between coats. Oh yeah, never pour leftovers back into the main can...bubbles and dust.I digress a bit here...I drag out my rakes, hoes, shovels, etc. when I am vernishing at home. I brush out the leftovers onto the handles, makes 'em last longer and reduces blisters.Speaking of brushes, invest in a good china bristle brush... we're talking at least $20 for a 1.5" brush. Cheap brushes simply won't get the job done.When applying the finish stroke in one direction only, wet to dry and don't go back over where you've laid the coat down. This is why a top finish requires 6-12 coats sanded between each coat. When you drag your brush over a section that is starting to skim over, you introduce new bubbles and you interefere with the finish's ability to flow out the bubble initially left there.You get the idea now why it is easier to 'show and tell' than trying to write out forty years of experience concisely.