No one likes mold. It’s an unsightly and often smelly problem that many spa owners encounter during the course of hot tub stewardship.
Because hot tubs are such a warm and moisture rich environment, many owners end up having a mold and mildew grow on porous surfaces like the cover. It may just start off as a stench issue which you may be frustrated to find doesn’t go away, no matter how perfectly balanced you get your water chemistry. It may rapidly grow to be a visible nuisance; if you start to notice black and green growing splotches on the underside of your hot tub cover, you've got mold.
It’s important to not just clean the hot tub itself on a regular basis (see a hot tub maintenance schedule here), but to take care of the cover as well. Dirty, dust, and mildew will do its best to grow on the cover, which may not seem like a big deal seeing as a cover is meant to catch all that stuff. However, if it becomes more and more of a problem, it will ultimately affect your hot tub water.
Part of the issue may be that your water isn’t properly balanced. If you’ve chosen chlorine as your hot tub sanitizer, you may need to up your levels so that mold can’t grow in that environment. You may just need to perform a hot tub shock treatment to get rid of the smell.
Small amounts of mold growth can happen to anybody, but you can help prevent it by routinely cleaning your cover. Do this by wiping down the entire cover with a cover-specific cleaner at least once a month. You can also substitute this with a mild household cleaner like a teaspoon of dish soap diluted into a bucket of warm water. Just make sure you thoroughly rinse the cover off afterward so as to not affect the water chemistry.
If you have a minor smell problem, be sure that it’s not coming from your filters (which you should also clean once a month in filter cleaner) or from unbalanced water first before concerning yourself with the cover. But if the cover seems to be the undeniable source, it might be time to break out the bleach!
Only use bleach if you have visible evidence of the mold. Use a diluted bleach and water mixture either in a bucket or spray bottle to get rid of black or green mold colonies that may pop up on the underside of your hot tub cover. This will help to kill the mold.
You may also consider removing the vinyl cover and wiping down the internal foam core to make sure all mold is removed. When you are done with the bleach solution, make sure you rinse everything off thoroughly and let things air dry in the sun before putting it back on the hot tub.
Step three is a lot like step one: to prevent the hot tub cover mold from becoming a problem again in the future, make sure you clean your cover regularly (once a month, ideally) using cover cleaner solution and water. Not only will this extend the life of your cover that you’ve invested in, but it will keep everything mold free.
If your problem seems to be unrelentingly chronic, consider a visit to your spa dealer to ask for advice. They're always happy to help!