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Hi Observer, What tub do you own, where do you live and what brand covers are you using? I just picked up a "cover" for my cover. I had problems with sap and tulip poplar "flowers" dropping on my old Coleman spa and getting the cover dirty/sappy. So, with my new Artic Klondiker, I decided to get a cover to protect the cover. It cost around $60 and I'll be putting it on, tomorrow. If the birds attack it, it is a lot cheaper to replace than the insulated cover.
Do you know this for a fact tman? Have you seen a smart top in person? They claim to be as energy efficient as a regular cover. I have wondered about that with the smart top myself.