Sanding sealer isn't really meant to fill the grain, it's there to stop finishes sinking into the wood. By its nature it may help fill small pores, but you're better off with a dedicated grain filler if the surface is fairly uneven, as ash can certainly be. Of course some sealers also have fillers in them as well so are dual purpose.
Plus, as Bakersdozen says, grain filler can be used to highlight the grain.