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Why am I getting Mold on my Canvas Top and How Can I Keep it Off?


Milton asked:


I have a 330 Sea Ray with full canvas. I have mold that grows on the inside of the canvas cover. I have cleaned it with dawn dish detergent with a little bleach and it come back in a week or two. What is the best way to get rid of this black gray growth and to keep it gone?



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This actually three questions:

  1. Why am I getting mold on my canvas top?
  2. How can I get rid of the mold on my canvas?
  3. How can I keep the mold from coming back on my boat?


I feel your frustration. Try as hard as you will, there seems to be no end to it. In some instances, you can stop the growth, however in other areas like Florida where it’s always hot and humid, the best you can do is minimize the growth, protect the surfaces on your boat from permanent mold damage and slow down the growth. In all cases it will require constant vigilance and a regular maintenance regime with the right products and methods.


  1. To start with, it’s important to understand why you’re getting mold in the first place and what’s causing it. Mold and the study of mold is involved and complex. It’s an important and necessary part of life on our planet as it serves to digest dying and decaying matter but it would be wonderful if we could just keep it in its place. Without mold we wouldn’t have mushrooms, certain cheese and penicillin. It’s just that no one wants a mushroom farm in their boat.


Mold needs a number of factors to incubate and grow. Think of it as a circular chain with a number of links. If you break one or more of the links, you break the circle and stop the mold. Reintroduce them and it comes right back.


Mold requires: 1) Mold spores, which are omni present on our planet. 2) Moisture; mold can’t grow in a desert. 3) Warmth; when it gets too cold, mold hibernates until it gets warmer and very high temperatures will kill it. 4) Darkness or shade; mold doesn’t grow in direct sunlight. 5) Food; this can include dirt, soap scum, suntan lotion, human skin and sebum and anything that’s dead or dying. 6) Calm air; mold needs oxygen but also calm so the spores can alight and germinate.


  1. To get rid of the mold, you need to break the chain. The inside of your canvas boat top is providing the ideal breeding ground for mold. I’m guessing that your boat top is made from Sunbrella, which is an acrylic canvas and can withstand a mild bleach solution. Some canvas can not and bleach can cause permanent damage. The bleach is killing the active mold on the Sunbrella and washing is removing it, however the detergent or soap that you’re using is not free rinsing and is leaving behind a residue, which is food for mold. Think of soap scum in your shower. If you don’t eliminate it, soon you will have mold. Washing the shower walls with more soap or detergent won’t eliminate the mold, you need a special cleaner for that. All the other ingredients are present in your boat to get the mold started again, quickly.


The gray, black, green or brown that you see on the surface is the bloom and not the root. As the mold blooms it gives off more spores into the air so that it can continue to propagate in surrounding areas of your top, boat seats, carpet, even the non-skid surfaces of your boat deck. When mold starts to grow on vinyl, the root starts to penetrate and it starts to eat the vinyl below the surface. The permanent mold stains that you see on vinyl seats as well as PVC and Hypalon inflatable boats and RIB’s is actually the digestive residue or mold poop. The problem is that removing it or killing it with bleach will cause permanent damage to the substrate as you wind up removing part of the substrate.


First you need to clean the Sunbrella and remove any residue soap scum. To do this, mix up a solution of 1 cup of chlorine bleach and 1 cup of Fabri-Klean in a gallon of warm water and transfer it to a spray bottle. You’ll need to remove the top so you can clean both sides. Spray it onto the fabric and brush it with a soft bristled brush such as a car wash or paint brush, trying to get it into the fabric. Flip over the top and repeat the process. Allow it to soak for about 15 to 20 minutes. Now rinse well on both sides with a high-pressure stream of fresh water from a garden hose or pressure washer at no more than about 1,200 psi. Wipe it with a towel and while still damp, reinstall it on your boat frame and allow to air dry. Drying while stretched on the frame will prevent shrinking. Leave the windows out or open to allow good air flow to the underside of the top.


Fabri-Klean is a free rinsing, deep penetrating, concentrated, cleaner that will deep clean the fabric without damage to the canvas or threads and will emulsify the soap scum so that it can be rinsed away.


Select a time when there’s no rain forecast for a couple of days and it’s going to be warm and sunny. When the top is dry spray Canvas Shield onto the top, while it’s still on the frame, to prevent shrinking, making sure that the surface is evenly wet Allow it to dry. This normally takes a couple of hours. Spray on a second coat and allow to dry in the sun for a minimum of 24 hours. You only need to do this on the top, not the underside, as the Canvas Shield will penetrate between the threads and coat them rather than just sitting on the surface. This will ensure that your canvas boat top is waterproof, eliminating or reducing the moisture requirement for mold, plus Canvas Shield won’t support mold growth.


A single application of Canvas Shield can last for several years but many boat owners clean their tops and covers and reapply it annually, as a form of preventative maintenance. Reoccurring mold will eventually cause permanent damage to boat seats and other surfaces. It can also be used on canvas boat cushions, awnings, patio furniture. It waterproofs and stain-proofs without changing the color or texture of the fabric and still allows it to breath.

  1. Cleaning and protecting the top is only the first step in reducing or eradicating mold on your boat. The next steps are to eliminate or reduce as many of the ingredients in the mold chain.


Since you’ve been getting mold reappearing on the inside of your top on a regular basis, that indicates that you have a concentration of mold spores in the cockpit and possibly the interior of your boat. It’s time to give these areas a thorough cleaning and then protecting the surfaces with protectors and polishes that won’t promote mold growth. This means free rinsing cleaners and synthetic polishes and protectors. Soap and detergent will leave residue that is food for mold. Natural waxes are also food for mold.


Fabric cushions and pillows can be cleaned with Fabri-Klean and protected with Canvas Shield as above. Vinyl seats can be cleaned with free rinsing EZ Vinyl Cleaner and protected with Vinyl Guard. EZ Vinyl Cleaner is tough on dirt but won’t rot the threads or leave pink spots like household cleaners and Vinyl Guard can protect your seats for up to 6 months. It’s not greasy or slippery, won’t transfer onto your clothes and will protect against UV, food and beverage spills, suntan lotions, will protect your vinyl against mold penetration, until you can get it cleaned away and each application will replace the lost plasticizers to keep your seats supple and fade resistant.


Use Boat Clean Plus to replace soap for shampooing your carpets and washing your boat. It’s free rinsing and environment friendly so rinsing into the lake is no problem. Use Boat Scrub with Hypexine to clean hard surfaces such as the vertical fiberglass surfaces in the cockpit and protect them with Premium Boat Shine, which is all synthetic and can last up to a year. You can also use Boat Scrub to clean the interior of your refrigerator, stainless steel sinks and fiberglass walls in the head. Bedding should be removed and laundered on a regular basis.


If you’re getting mold growing on the deck, especially the non-skid surfaces cleaning, sealing and protecting the deck with the Sure Step Deck Protect Kit will stop that, eliminate another source of mold spores, make your decks super easy to keep clean, restore the new boat shine to the decks and give you added traction.


A thorough cleaning and protecting like this will go a long way to eliminating the mold spores and food sources and therefore breaking the mold chain.


Some things like controlling the ambient temperature and daylight are simply beyond your control, but there are still a few other things you can do to eliminate or at least minimize mold growth. To control moisture, you can install a dehumidifier or use desiccants. Just beware that desiccants use salt to draw moisture from the air and that may be corrosive to metal and electronics on your boat.


Create positive air ventilation. Keeping the top tightly buttoned keeps the rain out but creates a calm air environment and harbors humidity. Try to get positive ventilation in your cockpit or your entire boat. You can get you local canvas shop to install ventilator cowls in your top that will allow air to flow through but prevent water from coming in or you can slip plastic foam around the base of the top to prop the bottom out a few inches in several areas to get air flowing through. If it’s impractical to keep hatches or port holes open, you can get sun powered ventilator fans that are installed in the hatches. These are maintenance free, don’t draw on your battery, keep the weather out and give you positive ventilation inside the cabin which can flow into the cockpit.


In short, keep your boat clean, dry and well ventilated. I know that this process sounds like a lot of work and possibly overkill, but compared to weekly mold removal, plus the hazards to you and your family breathing in mold spores, which is not healthy, in the end it will be a huge saving of time and you’ll be able to breath easier, plus having a clean shiny boat will give you greater pride of ownership and add value to your boat when it’s time to trade  up or sell.


Thanks for your question,

Captain Aurora
Richard Kittar

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