Interesting discussion here!
From my experience with slightly thinned spot putty, it does a very nice job in those cases where you want something with a bit more build to it than the typical filler primer. I sometimes dilute it just enough to make it brushable. Then I brush a coat of it on the surface, which as mentioned by others above is usually already primed or sealed. The thinned putty tends to dry with less shrinkage/cracking effects than the unthinned putty. Keep in mind, when working on a warm day, the unthinned stuff from the tube dries out so quickly that spreading it on a surface can be challenging. In the winter, not so much, and using a credit card type spreader as squeegee works much better than in summer when it's hot out. But it is definitely not a major gap filler or anything like that... it'll cover grain but not major cracks.
Also, an example of the dampness getting through filler primer happened last week when working on my Adfecta. I had sanded off much of the filler primer not in spirals, and in doing so had scratched the glassine. The damp (not wet, just a little damp!) sandpaper/sponge imparted enough moisture to the tube that it swelled a bit in spots. Some of these went through existing filler primer to the surface below (which had been previously smoothed lightly, scuffing the glassine). Glassine is just cellulose, after all, not plastic, so it presents no moisture barrier, particularly if it is roughed up increasing its surface area. In the end, it wasn't a big deal. The acrylic sealer I put on did swell the areas again a bit, but they shrank back as it dried. By the time I got to the top coat (again, using acrylics) all was well.
Marc
