Getting a smooth finish with a water-based coat like polycrylic is easier than with oil-based polyurethane (which is prone to forming blobs). However, there are still some things you should be doing to ensure a smooth-as-glass finish. Using a high-quality brush will prevent your polycrylic from forming globs or streaks as you apply it. Make long, consistent strokes with your brush that go with the grain of the wood. Smaller strokes that go against the grain will leave your finish bumpy and bubbly. Only apply a thin coat at a time, making sure that it is even along the whole surface. If the polycrylic is too thick to start off with, add some water (around 10% of the volume of your polycrylic) and stir it in. This makes it easier to apply and reduces the risk of an uneven finish. You should never sand a surface coated with polycrylic, as this will leave it rough and covered in scuffs.