I have been struggling with a piece to get the finish to look smooth.

What kid of piece, Dan and what kind of finish. There are millions of combinations out there.

Not sure what the material is? Concrete, wood, painted wall? Can you please advise here as if you were responding to your own question? Thanks.

What type of finish are you trying to do? Stain or paint? What was the original finish? Sometimes if I'm having trouble with any finish, it's because I didn't prep enough, like sanding. Sometimes it's the product I'm trying to use. For example, it is going on too thick or thin, or as recently happened, the sealer discolored the finish. If your piece was prepped completely and all residue removed, check that you are following directions. If prep is good, surface was clean and ready to refinish, it may just be what you're using to finish the piece isn't turning out as advertised. Hope that helps.

Before the first coat of any finish goes on, you should sand with the grain, starting with 100 grit then 150 then 200. Remove all sawdust before applying the first finish coat.

You have to make sure the finish is completely dry EVERYWHERE. Otherwise if the finish is still tacky, The next coat will take much much longer to dry, as in days.

Oftentimes the first coat of any finish swells and lifts tiny slivers of wood resulting in small rough nibs that you can see and feel.

Sanding with 200 grit after the first coat may remove some of the finish, but it also removes the nibs. After adding a second coat, if there are nibs again, sand again.

Always add a coat of finish after sanding.

Use a good multipurpose brush, apply finish with the grain. Don't shake the can of finish, otherwise you will get tiny bubbles that will translate into a sandpaperlike finish. Stir the finish every twenty minutes or so as it may settle.

Good luck!

What type of roller or brush are you using? After sanding with progressively higher grits, vacuum and use a tack clothe to remove any grit. I like a smooth 6" foam roller and a new mini tray to apply and then I quickly use a badger brush to drag off any texture.

When preparing your top coat, be sure to not shake the poly can as air bubbles form in water-based top coat. Instead stir the can slowly with a paint stick. I prefer Varathane brand.

If your seeking the high-gloss glass-like finish, you use a epoxy bar top finish.

using a super fine grit sandpaper between the coats, either foam or expensive brushes and applying a topcoat can help make a finish smooth