Hey Aerody, man the MP 4/4 looks great!! Only thing missing is your tire markings. Can't tell'ya how many kits I've had to order spare tire decals for.
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You're definitely not alone there.
As far as the black primer... true, I wouldn't have normally gone with black as my base primer for a white car. I wanted to try out the UMP primer in the pic there (really just re-branded Badger Stynylrez primer) as I have always used Tamiya primers. Depending on the paint it doesn't make too much difference. Obviously if you are using a metallic, pearl, or opaque type paints you would choose your base primer more carefully.
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I used decanted TS-26, which is Pure White, and the opacity of that white covers the black completely. If the car was Red or Yellow, etc., that would be a different story as those colors change "shade" as you lay them down. The white goes on the black, white. Anyway, it's best to test on spoons, spray a bunch of plastic spoons with different primers and then you can test your paint.
Here below is the BMW Z4, painted in TS-26 over Tamiya gray surface primer (lacquer).
And here is the Merc AMG, painted in TS-26 over the black UMP primer (water based acrylic).
You can see, they both have come out "technically" the same color white. It's because of the paint, the TS-26 covers well and does have any opaque properties to it - had this been a "metallic" or "pearl" paint - or even a red/yellow/orange color, than the shades would've been slightly "off".
Generally if I'm gonna paint a car red, say a Ferrari F1, I will use a pink primer (or make my own...). Anyhoo, could go on for days... painting is one of those personal things and is best "tested" before "committed".