The sealer might be an unnecessary step, signs usually just hang on a wall and the acrylic should hold up just fine indoors. Acrylic will not work outdoors and neither will Polycrylic. The addition of the sealer will make for a nicer sign though. I'm not sure why the Polycrylic is smearing the acrylic. How long are you waiting to apply the Polycrylic? Are you using a stencil or a mask? How are you applying the acrylic? My guess is the acrylic isn't completely dry and the Polycrylic is breaking it down. I've found some brands of acrylic take longer to dry than others.