don't sand it till you're ready to start prepping for paint,as it been said before,most primers usually sand better if you wait, if you're leaving it in garage,just throw a car cover on it,DO NOT leave the car outside,some primers can absorb moisture if left exposed in weather,dew ,condensation,sun will break it down,since primers do not have UV protectants like topcoats
most bodymen start blocking with 180 grit, I usually start with 120,since the primer I use is very high build,so there is plenty of it to work with,I finish with 320-400,or 600 depending on the color,since light metallics are not as forgiving as darker or solid colors and will show sandscratches, especially with watered down PPG paint
good luck
Stefan
Stefan