can show us in pictures of your current masterpiece of the screwed up?

i can only see so much from the picture, and i can't tell the overall condition of the spray paint, but it seems to me that using high grit sandpaper will smoothen out the affected part (is it because you spam spray that part so much that it bubbles up and then burst after it dries off?)
now you know that spraying needs patience my ducky plate took me like 2-3 hours to spray per side, i did like 1 light coating every half an hour i think, and i understand the urge to spam, i felt that a lot when spraying my plate

i only have spray experience of one keyboard, other spray sifu might give you better tips and pointers...but nevertheless, all the best! white case is awesome! just finish what you started

Simply use 600 grit sand paper to sand it. Followed by 800, and then 1000 for final sanding. Wet sanding is very important and it help a lot. Instead of using and hurting your thumb, just wrap a block with sand paper and sand it. The most important part is painting. Not to close because it can be melted. Not to far because it will be faded and matte. Not too thick because it will crack and take long time to dry. Not to thin because will easily get scratched.

Maybe if you want to experiment with cracking finishing design, paint with two layer of color. The cracking finish is very sexy on headphone amp but I wonder if it will look nice on keyboard. I knew that cracking finish casing is awesome but hard to take care off.

Why do you want to spray the case in the first place?
Because you want a white case? or mainly because you interested in DIY?

Do you find any filco replacement casing you like in taobao yet?

If you know how to do and you should have do it right. Your impatient is wrong. If you spray too thick, that will be a problem. That's why it's the best to paint with multiple thin layer. Easier to cover and make them equally painted. I'm willing to help but too bad I can't.