There are a lot of things you can do wrong when mounting watercolours or acrylics painted on paper. So I was pleased to find John Salmon´s write-up on the subject. He tells us in detail what to do and what to avoid.
E.g. John says that acid-free water soluble gummed tape should be used to attach the paper to the mount. Using acid-free material throughout ensures that the acid does not ruin your painting over time. That advice also applies to the backing board.
Some time ago I also wrote about my personal experience.