I'd say its impossible to get a matt finish whilst only painting the inside of the body, I think you have two options
either as pointed out, paint the inside black and then use fine sand paper and rub down the body on the outside until your happy with the effect
or paint the outside of the body with black, but this opens up the possibility of chipping, cracking the paint and ruining your body.
Personally if I was to try I'd say sand down the outside, by no means a quick way of doing it but seems like the best way to me.
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