American frosting advice please

[OPeaches]

Posted this in baking but realised it was in the wrong place, so sorry if you've seen it already.

Am planning on using American frosting on a rainbow cake I'm making so I can get a nice bright white finish. From what I've read, i.e. a quick google, I need to work fast to get the cake covered as the icing sets quickly. So, two questions:

How quickly? How long do I have before its unworkable?

Do I need to crumb coat first with buttercream?

TIA

OP can you give more info on the frosting, ingredients, is it from a packet? What I have always called frosting doesn't actually set hard.

Don't have the recipe to hand, but you whip egg whites, make sugar syrup then whip them together for 15 minutes. Does that make sense?

Or, do you have a simpler way of making snowy white icing? My way sounds v complicated!

For an 8" cake. the following are US cup sizes (8 fl oz) 1/3 cup solid vegetable fat, 2cups icing sugar. Plus pinch salt and 2 tablespoons cream.
Cream fat and salt, stir in the sugar, then the cream. Beat well. Add more cream or sugar if necessary to get spreadable consistency.

I looked in my Fanny Farmer, She was the American Delia, long before Delia, honest. Your method is called Mountain White Frosting. The one I use is Confectioners Frosting.

Auto correct! Fannie Farmer

Thank you - will try your way I think. I had no idea you could use vegetable fat to make icing.

Why not try italian meringue buttercream?

The Italian one sounds lovely. Will need to invest in a thermometer first though.