A lot of it is going to depend upon the fiber content of the existing furniture. In general, I'd say it's a bad idea. It may not take to the existing fiber(s) evenly, might transfer off onto clothing, might not be colorfast to light, water or abrasion, and might just slough off.

I am really skeptical of the below -- all 5 star reviews sounds like a shill to me.

At any rate, try on a pillow that you don't mind loosing or arm covers before starting out on the center inside back.

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