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I am in the process of winterising my inflatable hot tub.

The first thing I noticed was that the inflatable "body" is heavily discoloured by mould that is growing inside it:

Mould

I assume that the cause of this mould is the excessive amount of water that was present inside the inflatable ring itself. By various means, I have removed around 3-5 litres of water from within the inflatable cavity.

I have no idea how this water got in there - my calculations give a maximum amount of water vapour in this volume as around 25 millilitres. But that's another story.

I'm looking for ways to 1) dry out the remaining air inside, 2) kill the mould. Bear in mind that I have a single inflation hole - around 25mm (1 inch) in diameter - through which I can operate.

I've tried warming the surface with a heat gun to get the water droplets to evaporate - but this is ineffective and risks melting the plastic.

I've tried deflating and reinflating a few times, in the assumption that it will replace with drier air each time, but the condensed droplets remain.

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I've had good results from a solution of water and white vinegar, although other types of vinegar will work, including apple cider vinegar, but I suspect the best results are white vinegar. I've used 100% vinegar, and also 50-50 vinegar/water. I've seen at least one online source suggest that the amount of vinegar can be as low as ten percent.

Pour a large amount of the solution inside and while the solution is in contact with the area of mold, squeeze, manipulate and massage the plastic to dislodge the mold from the surface.

Additionally, one can use bleach/chlorine solution, but it may be damaging to the material of which the tub is constructed. Vinegar is not going to damage the plastic.

As you will be adding liquid to the inside, you'll want to perform the cleaning prior to the drying process.

For drying, you can insert a small diameter hose into the walls, ideally as far away as possible from the opening. It will be necessary to have a continuous source of airflow into the hose, which must be dryer air than that inside. Warm air will accelerate the drying, but that can be challenging and is dependent on your resources and not necessary to accomplish the goal.

As the new, dry air is forced inside, some of the moisture will evaporate and travel to the exit.

Expect many hours for the inside to become dry, perhaps days.

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    Given the hours that I've already spent removing the water, I am loathe to go adding white vinegar at this stage. I think I will probably try this next year though. One of the many things I tried was to blow air through a small tube from my blower-vac - heated by the heat gun. This was hard work and achieved nothing in about 30 minutes, but If I was able to leave an arrangement like this running for a number of hours I assume it would start to do something.
    – Lefty
    Nov 17 at 18:35
  • Without the anti-fungal, the mold may remain and may return. You'd certainly be able to dry the inside with an in-and-out airflow system. Heat will accelerate the moisture removal, even if only applied to the outer surface.
    – fred_dot_u
    Nov 17 at 20:52
  • Yes, I understand. Obviously, the mould will still be around, but it is just an aesthetic problem, rather than causing any damage. Interestingly, the "Visible" parts of the ring are mainly free from the mould - I assume this is due to the raised temperature and/or daylight penetration in these areas. It is only the dark/cold areas where the mould thrives, as can be seen from the photo - the uppermost surface of the ring is clean.
    – Lefty
    Nov 18 at 14:56
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Here's your Hot Tub Hero Hack:

There are two steps to take.

First: Most all mould removal involves use of liquids or gas (such as ozone). Vinegar (white, undiluted) should give you optimal results according to most sources available online as you cannot also scrub the interior surfaces. Bleach (Sodium Hypochlorite) is less desirable for several reasons also given online. Commercial fungicides are also available for household use.

After the mould reduction treatment, removing the liquids remaining, will be less of a challenge using dehydration crystals.

There are various desiccators, desiccants, and moisture absorbers to pull practically all the water vapour from a volume of air efficiently. Some kinds of desiccants can be rejuvenated by baking in an oven. Your heat gun may also work.

The tiny access and plastic construction of the tub IS a challenge. You will want to fashion a long sleeve of nylon or some thin porous fabric filled with desiccant for your job.

Drain as much liquid from the hot tub as you can. Lower the filled 'tube' into the tub—holding the end so you can pull the saturated crystals out—and seal the opening. You may wish to tip the hot tub so your dryer-filled sleeve and the water vapour are closer to each other. You may need a couple of sessions to satisfy your requirements. You can do this at any time for minimizing water accumulation; but, the off season is probably best for maintenance.

Wait. Repeat as needed.

Good luck.

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    Thank you @Stan. As I said in my comments on fred's answer, I will try various liquid cleaning treatments this time next year before draining it down. Interesting you mention about a "sleeve" of desiccant - I had a similar idea myself just a couple of hours ago and ordered some sachets of silica gel. My idea is to connect them into a "chain" somehow, then feed the chain into the cavity, re-inflate the ring and leave the gel to do it's work for a few days, regenerating a few times if necessary. I'll report back how well this works.
    – Lefty
    Nov 18 at 18:10
  • Yes. I was more interested in the removal of the extra moisture than the mould for which there are some products as well as the tried and true vinegar. Don't forget waterbed conditioner although that's for enclosed standing water treatment. I suggested the sleeve to hold bulk desiccant material. There's also stuff called 'Concrobium™' which you may want to check-out. @Lefty
    – Stan
    Nov 18 at 19:00

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