Are you talking about Pastry Pride--nondairy whipped topping? Whenever I use it, I get some water boiling while putting the cake together. Once it's all covered, I pour the super-hot water in a tall heat-resistant glass/mug (tall enough for the icing spreader), then dip the spreader into the hot water, shake off excess water, and smooth the sides then the top. Keep re-inserting the knife into the water as needed. (I actually do the same with buttercream.....)
I use the biggest drywall spacker tool that I can find! I have one that is like a foot long or longer and use that. No knife marks!
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