There's a relationship...
More fat and liquid slows down crusting. So even if there's a lot of PS, you can slow down (or eliminate) crusting by increasing the amount of fat and liquid.
THe house buttercream from WBH has a full cup of boiling water added to the PS....so it doesn't crust. I haven't done the math on the fat/PS/liquid ratio, but it seems about normal.
But Sugarshacks buttercream crusts fairly quickly...it has less liquid and a lot of PS per recipe.
I'm sure there's a more technically correct way to explain it, but this is what I've noticed from using various recipes.
karateka is right. The more fat in the recipe the less likely it will crust, if ever. A good ratio is 1 cup fat to 2#s sugar. If that seems too dry increase the fat slowly - like 1/4 cup at a time until you are happy with it.
Better yet, just try SugarShack's and you won't have to worry about b'cream again
Thank you both. Here's my problem:
1. tried sugarshacks a month or so ago and it didn't crust at all. not even enough to smooth with viva.
2. used indydebit's BC for the first time last week and it crusted so much that it was hard as a rock and literally broke to pieces when the cake was cut. I posted a forum on that and got lots of help from indydebi and others....BUT I tried it again with a bit more crisco and a tad more milk...seems like I'm getting there but just feel so frustrated!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
3. I have to do a large football cake this week coming up and I just want easy, yummy, crusting but not hard and crumbly icing.
That brings me to my question!! So, I guess I will play around with it some more Maybe I'll try Sugar shacks again and see what happens. Indydebis is sooo much easier though!!!!!!
I use sugarshack's recipe and I love it! It crusts perfectly!
Before that I used the Wilton Snow White Buttercream recipe. It starts with meringue powder (the recipe come with the can of Wilton meringue powder). This always made a nice icing that crusts well too.
Good luck!
I'm not sure if you're up for trying a new recipe but I got this one from my Wilton instructor and it works every time for me. It crusts nicely but it tastes great and never gets hard.
1 kg (2.2 pounds) icing sugar
1/2 tsp. salt
1/2 cup water (I use hot so the salt disolves)
1 tsp. vanilla
2 cups Crisco (I use 1 cup Crisco, 1 cup butter)
In a small cup mix salt, water and vanilla, stir until disolved. Put into the mixer and add 2 cups p.s. and 1/2 cup Crisco. Beat until smooth (5 minutes). Add remaining icing sugar and fat alternating. Beat for a few minutes until really smooth.
I use the Viva method to smooth after I have iced and chilled the cake for 15-20 minutes.
(I only use it as a crumb coat under fondant).
I always add a little meringue powder to my icings which help it in crusting. Also the kind of shortening will affect the crusting -- zero trans fat won't crust but hi ratio shortening will.
Actually, the meringue powder helps your icings to not separate as much. It doesn't eliminate it, but it tends to act as a "binder". I'm sure we've all heard the saying that "oil and water don't mix"... right? Well, that's exactly what our icings are (generally speaking) If you've ever noticed your icing breaking down and having puddles, it's the oil and water separating. Meringue powder will not eliminate this, but it does slow it down.
I have recently started using the meringue powder in my recipe again. Between Crisco removing their trans fats and the humidity down here, I've needed to adjust my recipe. Mine crusts great, but doesn't "break" off the cake, as someone else mentioned. Not sure if you're interested in trying yet another recipe, but if so, here's mine:
Beat: 2/3 cup water and 1/4 cup meringue powder till it forms soft peaks
Add and blend until smooth:
2 sticks of crisco
1 Tblspn vanilla
1 Tblspn butter
1 Tblspn almond
(or 3 T of whatever flavor you'd like)
1/2 tsp salt (or a little more to cut the sweetness)
2 lbs powdered sugar
3 Tblspns corn syrup (to regain some trans fats)
This recipe has been working well for me. It crusts nicely, is easy to make, doesn't leave a greasy after-taste, and holds up very well in heat, humidity or cold.
Crusting or not crusting depends on your ratio of fats to powdered sugar or dry ingredients, but also depends on what type of fat. Butter fats need more sugar to be crusting than some other fats. Meringue powder will aid in the crusting, but you don't need meringue powder to have a crusting buttercream. Good luck and I hope you find something to work for you soon!
There are no dumb questions! 
Here's a little tip, as well. For recipes that use salt, use popcorn salt instead of regular salt. it is a much much finer grain, and therefore avoids the dissolving problem. I've been using popcorn salt for a couple of years now, and it works wonders.
I made Indydebi's b/c yesterday and let me tell you, I got so many raves on the taste. It is awesome.
The only problem that I had was getting it smooth. I iced a sheet cake and I truely do have problems with my corners, but not generally on the top. The one thing I noticed was that it crusted very quickly. I like the smoothing technique of sugarshack, but by the time I got the caked iced down, the top had already crusted.
I guess I should have smoothed to top before doing the sides. I don't have a problem with round cakes, only square and sheet.
Her recipes does call for less shortning then sugarshacks, so I'm guessing that is why it crusted really quick. I'll try it with a little more shortning next time. Both indydebl's and sugarshacks recipes are to die for. They both are a "10" on a rating scale to me.
I use the Wilton bc recipe (usually...I have a whipped recipe I use sometimes too) but I use half butter and half shortening (not Crisco) and then I just add milk until its the consistency I like (usually more than Wilton calls for).
I like it so that it does not crust very fast, so that I can smooth it nicely with my spackle tool before it gets crusty. I have had issues with using the Melvira method and Viva method lately (I tried showing the students at a Wilton class, using full Wilton recipe, and somehow it failed me!) so I just make it soft enough to smooth fairly well at first...and then I can touch up an hour later or so if necessary with Melvira method.
Meringue powder does not make icing crust... AND regular shortening WILL crust just fine.
Its all in the science of the ratio of how much sugar versus how much fat. None of the magic of those who are selling hi-ratio shortening and meringue powder want to get you to believe.
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