Unfortunately, WSF is grainy all the way through since it's built up from a powder base so straight sanding will just shave off layers of the same rough consistency. You can apply layers of varnish, paint, or some other sealant and sand that as it fills in the 'pours' but that won't work on your model unless you want to do some seriously detailed and time consuming sanding of every crevice of the intended model texture.
There is a method that some have used that involves mixing superglue and acetone together, then submerging the model into the mixture to coat it, and removing it so the acetone will evaporate and leave a layer of superglue around the outer surface that will be relatively smooth. It still works better when you can sand after the fact and either way, you'll lose some finer details, but it may be your best bet. I haven't personally attempted this method though so you may want to wait for someone who has done it to chime in with advice and specifics on the ratio of acetone to super glue, etc.
Others have used floor wax or similar compounds to work into the surface but again, these require sanding after the fact.
If you're planning to paint it anyway, I've found that a few layers of paint and varnish alternated will smooth out the surface with minimal loss of finer details. Here's a model to which I applied two layers of varnish, 2 of paint, one of varnish, one paint then one last layer (or two, can't remember) of varnish. This protects the base layers of paint and smooths out the surface considerably. For frame of reference, this model is a 1:5 scale Wii U, about 10mm high - so if the scale is larger on yours, it will look that much better.
Truth be told, I didn't take much care in ensuring a clean work space or that the layers were super consistent, so you'll have better results should you be willing to put in the time where I was not.