I wonder if it is actually possible to over-apply thermal paste. I once made an experiment: I put a drop between two acrylic glass parts from two CD cases, and gently squeezed them together. After a short while, it spread out so thin that it was practically transparent. From then on, I believe that excess thermal paste simply squeezes out at the edges, where it does not do much harm if one has the foresight not to use an electrically conductive type.
I also never try to apply it on the complete surface. I believe this is an easy way to trap bubbles. If I use it on a round surface, I put a drop in the middle and let it spread out on it's own when assembling the part with a little pressure. When I use it on a square surface, I apply it in an "X" shape, from where it spreads out, and so on. I always try to think about the way the thermal past would spread out on it's own without trapping air bubbles while covering the area completely, and apply a pattern accordingly.