Problem - how to make a model solid =(

Hello guys,model now is 100% printable i test it on prusa 3d Priner in my work(I don’t have my own 3d printer)=)
Thank everyone for help i will remake fin witch follow me tool (thanks for hint=)
And for explanation how short edges works=D
In a few days i will add here finished model if someone want to use it anywhere.
It’s Bmw e60 front lamp back case=)