mildew on underside of cover. Why?

[mscdman]

The last few months I’m seeing mildew fairly frequently on the underside of my cover.   
 

im running my ho tub at 85 F over the summer - like a pool - but I have it balanced and properly brominated. 
 

i wasn’t seeing this all fall and winter when I had the tub at 102.  What could be causing the mildew?

ive been straying mildew remover on it to clean. Bad idea?

[RDspaguy]

Covers need to be cleaned from time to time. With nearly constant moisture, darkness, and warmth it is a great place for nasties to grow. Chemical fumes trapped under the cover can prevent this, but will ruin your cover in the process. So the fact that you have mildew is a good thing from that perspective.

Keep bromine levels where they should be and wipe down the cover with bleach once a month or so.

Not sure what "mildew remover" is made from so can't say if it's good or bad.

Maybe @waterbearhas some idea.

[mscdman]

Mildew remover is basically bleach.   
 

I hear what you are saying, however why would I have never had this issue for 6 months with the tub at 102?  Hmm…you know what?  I bet it’s because I’m not using the tub at all this summer and it barely gets opened.   Duh

[ratchett]

On 8/12/2021 at 8:51 PM, RDspaguy said:

Covers need to be cleaned from time to time

I've been using dish soap and water on the exterior cover (then dry and apply 303 Protectant).  But is bleach the best option for the inside cover? Or is that too aggressive on the vinyl?

So far I've used a heavy concentration of cleaning vinegar + water + splash of d-limonene citrus extract

[RDspaguy]

I always used bleach. Just rinse it off.