It is my opinion and observation that applying fairing compound under glass work is not always the best. Shrinkage comes into play when you have a disproportionate amount of resin to the faring balloons that's meant to float across a surface making it easier to sand. By far the thicker filler materials are harder to sand and can cause some craters in the wood, which is normally softer than the thickened resin. A mixture of fairing compounds and a final blend of cabosil is what I normally apply if I have any real deep dish on the woods. Then if need be apply a skin coat of fairing after sanding the harder materials. This is actually done with a soft pad and grinder to knock the surface skin off. Then go back with a DA. or random orbiter.