I've always just figured that for buttercream to crust nicely, it has to be rather thick. This has caused me some problems before, because I find it hard to apply to the cake nicely.
Then I remembered that what decides if buttercream crusts or doesn't crust, is the sugar to fat ratio in the recipe. Does that mean that as long as the ratio is right, you can thin the bc to the consistency you like (using milk etc.), apply it to the cake, and then find it crusts well? Or does the fact that you add more milk "mess up" the sugar to fat ratio somehow, and cause the bc to not crust so nicely? Any thoughts would be most welcome, thank you :-)