[brass_monkey93]

I have been attending school for a year now for bodywork, hopefully this will help you. Your first question about sanding the ground effects: Since it is a type of plastic it won't rust, so you dont have to use a very coarse sandpaper...I recently redid my front bumper and I used 220 for the whole thing. You don't need to take all the paint off, just scuff it up good. Primer will actually hold better when it is applied on paint than anything else. You dont need to go any finer than 220 when preping for primer, to you it will feel very smooth, but there will be sand scratches after you apply the primer, this is fine because you will need to wet-sand the primer down and that will take care of the scratches. I would NOT use a DA sander on ground effects, do it by hand. Only use a DA on a hood, door, roof, hatch-until you are familiar with it, they can eat through paint very fast. Now on to your question about pinholes in your plastic body filler (bondo). Those will usually appear because you are mixing the hardner and filler wrong. First thing you just need to run one line of hardner across the top of the filler, this will be more than enough. When you mix it use a scraping motion, if you have been using a stirring motion this is most likely how you are getting pin holes. Right before you apply the primer to the car, wipe down with a "Wax and Grase Remover" this will help eliminate lifting in your primer.