I just want to ask if i want to repaint my PSU , do i need to sand it?

advisable to sand... and of course to dismantle the electrical components out of the case

make sure to scrub it using two sand paper size, as a habit

smaller grit size/rougher one to remove the painting layer while
the smoother one to smoothen the surface

yup,recommend to scrub it for good finishing,else the paint might be peel off later

Thanks all ! Will do