ok Nitro your not priming it up properly you need to put enough primer on it so you have a margin of not sanding through, the body is ASH? so grain fill is paramount before you shoot the colour coats Ash being so grainy it will suck up quite a bit of primer its a common mistake to get a nice shiny flat finish is all in the prep if your only learning don't be scared to grab the sander and start again, as the paint drys it will fall back in to the grain and then you need to have enough paint to be able to sand that out. when you spraying a full colour you sand differently you don't sand with the grain as all that will do is the surface isn't flat you need to do a circular motion with 120, 220, 320 the anything over 400grit. the good thing is your in the right place to get advice there is some decent builders on theses forums and most will help. I understand your English isn't your main language so translating maybe a problem but good luck with it you'll get there