What kind of brush should I use on my final coat of varnish?

[jenny]

I'm on the last coat of a walnut bookcase that I'm re-varnishing. I'm using Minwax's two-in-one Polyurethane and stain blend. To go with it I'm using a Minwax varnish brush. However, it almost seems like the brush is too coarse and is leaving lines in the varnish. When the varnish dries, it feels smooth but you can still see the lines in it. I want the final coat to be even and smooth. Would it be better for me to use a foam brush? Or should I find a different kind of bristle brush?

Thanks for your suggestions!