why not just use royal icing--it's already nice & white and i would mist it with water after applying to get that texture with the spatula--done

K8 is right, RI works great for this. A big advantage is when you are finished, you can just wash the RI right off the dummies which can then be used for another project.  Right in your sink; no worries about clogging your drain, just use really hot water.  I would not recommend that with spackle!
 

also -- you can sand it smooth ;)

Thanks for the replies! I am definitely open to using royal icing.  Will it hold up ok since I'm making it 3 weeks in advance? Also, reusing the dummies isn't a factor.  Since I'm in the wedding I've decided to just let the venue dispose of the cake at the end so I won't have to worry about it and can just enjoy the night with my sister.  I know indideby says her buttercream recipe hardens solid for displays. Would this be a good idea or is RI better? Thanks again! :)

yes--will hold up fine--treat it like a cake and keep it out of sun and weather of course

i'd use the royal myself because you can sand it to perfection

but i'm sure indy's icing would be great too

we only used buttercream to cover our dummies at one place where i worked

it has to be replaced more often than royal but three weeks typically is no problem

best baking to you!!! sweet labor of love for your sister!

RI will last for quite awhile.  I still have a 3 tiered dummy cake that I made for a class in September last year.  I have a clear plastic bag over it to keep the dust off but it has held up great.
 

Thank you both SO much! Super helpful info! You have definitely talked me out of my spackle idea lol.  Do you suggest any RI recipe in particular?

well i think for cake covering i'd recommend using meringue powder royal icing

as opposed to egg whtie royal icing

i often use the joy of baking website to remind me of the formulas

of course the recipe's on the can of meringue powder too--that you can purchase at cake deco stores

and hobby lobby type stores

So I chose to use the royal icing for my dummies and actually got them all iced today! :D  The RI worked perfectly for what I needed and dried very hard.  I used Antonia74's royal icing recipe minus the cream of tartar (my Wilton meringue powder already contained cream of tartar).  Here's a pic of the results...

How did the RI hold up for you? I'm about to make a dummy also and was thinking about using spackle, but found this thread before I started.

pieceofcake561: Spackle in a wedding shop display probably doesn't get snacked on. Spackle on a Wedding cake at an real live wedding, yuck. People, (& not just kids), have a huge tendency to put everything in their mouths. 

AHahah!! Agreed..this is a dummy for an upcoming bridal show &I would like it to hold up for display afterwards. It's going to be covered in fondant but I read that spackle under fondant holds up better. Didn't know if that was an accurate statement or not.

AThe royal icing held up perfectly. It went on easily and I had no issues w it at all. It also worked well to "glue" the tiers together. You can see the cake on my Facebook page: www.Facebook.com/MyCakeBakery