I think you have to sand the brushstrokes out between coats if you want to end up with a glass like finish at the end. This is a royal pain since the coats are so thin to begin with. There are a couple of tips to minimize brush strokes.
You can't brush and re-brush shellac like you do with an oil finish. Brush it on as best you can in a single stroke or two. The most important thing, but the hardest for me to do is to not keep messing with it.
I have a Taklon brush that I got form Homestead Finishing. It's fine bristles minimize brush marks. After each use, a quick rinse in alcohol is all you need. Let it dry hard; just redissolve it before the next use.
I think (but I'm not 100% sure) that Seal Coat already has some retarder in it, but shellac retarder will help shellac flow out better, minimizing brush marks.