You can probably sand them out, but you will definitely have to put additional paint on afterwards. Before you even consider sanding, ensure that the paint is 100% cured. I would wait at least 3 - 4 days (from the day you painted the model) before attempting to sand.
If possible, use wet/dry paper and sand wet starting with the finest grit you have (just the opposite of sanding down imperfections). If this gets the marks out to your satisfaction, then you're good to go. If not, move up in grits as needed until the brush marks are almost completely removed. Then come back down in grits to the finest you have. This will help remove any scratches from the heavier grit papers while finishing removing the brush marks. A major concern is that you will also remove any raised detail.
Good luck.