I finish off oak piece and finish feels like sand is in it - I used a fine oil brush and made sure the surface was sanded at 600 grit - wipe down with tack rag and still have a sand paper feel- so what gives?
Oak is famous for grain raise when varnished; you need to fine sand between coats.
Also important but under-advertised is the importance of applying coats that are appropriately thick when using water-based poly, too thin and the raised grain pokes right through.
Probably tiny tiny bubbles in your varnish / coating. You didn't shake it did you? Even if not, it's difficult to avoid bubbles in some types of finishes. Brush techniques vary by the type of finish you are working with for this reason. It can also be dust that settled on the finish before it completely dried. In many cases you must take a very fine fine grade of sand paper or emery cloth, i.e. 600 grit or higher, and lightly sand the surfaces between coats. On the final coat, assuming an oil based finish like polyurethane, you do a "wet sand" using something like lemon oil as the final step, with almost no pressure. There are some decent videos on YouTube available to show you techniques, but again, they vary by the type of finish you are using, so start by searching with that in mind.