The more important thing is to use a short stiff-bristled brush in a stippling motion to create the effect. A small-sized old oil-painting artist's brush, trimmed a little, works well in this role - the stippling is hard on the brush, so don't use anything too good/expensive.
Whether you use Mr Surfacer, Tamiya putty, fine-textured masonry paint, or even just plain old solvent weld (assuming a plastic model, of course), the stippling with a brush is what produces the distinctive sand-cast texture.
I found this short video here, which may help you:
https://youtu.be/mA5jUbQ9WQs