The dulness can be from a few factors, one it could be the technique in whcih you sprayed, have you used a clear finish? You shouldnt sand it down now after your final top coat but just cut and polish it , you should of just sanded it with a fine grit after the primer coat. Also it could be the mixing of paint and thinner could have been wrong which is why you got dullness but usually with arcylic laquer when after you cut and polish it you will get the shine and it will make it a smooth finsh