I miss my 'Dera!
That mottled paint effect is entirely due to the paint you are using. Remember well what kind it is and never ever buy it again. What I found that works well is Krylon Tough Coat, which comes in both flat black and hi-temp flat black. It covers great. Just plain good quality paint.
I tried all the hardware store hi-temp blacks and none of them worked worth a dang. So I went to an industrial paint instead.