I was not going to fill the rivets on the undercarriage cover skin, but I finally decided I would try. It's a little tricky filling rivets underneath the aircraft. I have tried a different method, which I will know whether it works or not when I sand the rivets. I filled a syringe with body filler and sqeezed the filler into the rivet head. It was very quick but it may not work, I will let you know if it does.