This thing is MUCH better to attach before the body is painted. It doesn't like to stick anyhow. I personally use the model master liquid cement, which is a consistency halfway between regular liquid cement, and tube glue. It allows a bit more working time ten regular liquid cement, and is still nice and tidy.
My way to attach it is to add a couple of drops of this stuff to the bottom of the mounts, and then get it REALLY close holding it with the thumb and forefinger. Make sure the thumb and finger hange over the ends, and pay attention. Get it lined up as best you can. Use your overhanging fingers as a guide to get it perfectly centered, and do minor adjustments, then carefully set it aside somewhere safe. Next day, use a fine brush and some liquid cement to reinforce the joints, and if you have the inclination(I personall did this on mine), hit it at the ends of the spoiler as well. Mine has held nicely since I used this method, and it's been handled quite a bit.
I'm letting my armor modeling sink in for the paint job, painting it WWII German Dark Yellow, and am going to put a couple of BalkenKruze decals on it for graphics. Yes I know I'm one sick twisted puppy.