Funnily enough, I've just finished spraying and polishing myself so I should be able to help you here.
Were the mirrors plastic or metal? If plastic, then there is every chance that they had a grainy effect as the original finish. This can only be resolved by spraying upteen layers of filler primer over them to fill the small indentations. Then sand down to a smooth finish and top coat.
If they are metal and the surface was smooth before you sprayed them, then the grainy effect is simply orange peel. To get rid of this, sand down the paint with a fine wet and dry paper (1000 grade is best, 800 is coarser and does the job quicker but you must be careful not to rub too much). The idea is to rub off the top layer of paint (the bobbly layer) to reveal a smooth surface beneath. After rubbing, the paint will have a matt finish. Don't panic! Just rub some T-Cut over the paint, let it dry to a haze and then buff it with a cloth. Its like magic. If you have rubbed the paint down to a smooth finish then the result after T-Cut will be a mirror reflection.
Do remember: don't rub too much paint when sanding down and T-Cutting as you'll go through to the primer. Obviously the more layers of paint you put on in the first place will determine how careful you need to be.
I've just done all of this on my camper van after a DIY spray job and the reflection/shine is perfect.
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