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 :-)

You can thin it as much as you want, within reason, and it will crust if the ratio is correct.

Thanks, guys. remnant, my buttercream is 1 part butter, 2 parts powdered sugar, a couple of teaspoons of vanilla extract and just a little bit of milk. I think I will try to use some more milk then, and hopefully it will be fine :-)
 

A2 Cups high ratio shortening 2 teaspoons butter flavoring 4teaspoons vanilla extract 2Tablespoons meringue power 20 teaspoons heavy whipping cream. More if needed to thin 2 pounds powered sugar

Thanks for the recipes, guys. But I can't use shortening - the closest thing we've got to that in this country comes in solid blocks, waaaaay harder than butter, and no matter how long you leave it on the counter it never reaches the consistency of the American shortening (Crisco etc). I did try it a couple of times when I first started out, but it's impossible to get rid of the lumps - our "shortening" is only meant for melting. So everything I do has to be all butter. :-)
 

So just use all butter.  All butter tastes better anyway.  The type of fat you use doesn't matter, it's the ratio of fat to sugar that makes it crust. 
 

Word!  All-butter is the way to go! :-)