DIY: Convertible Top Waterproofing

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Staff member
Dec 14, 2002
New Jersey
Of course no convertible top is waterproof, and as such needs to have its repellent replenished every so often. I decided to do it yesterday as the car had been sitting in the garage for over a week so the top was bone dry. Some repellent products apparently want a dry top to prevent moisture from being sealed into the top or fibers. Not sure about how that works, but I started with a dry top all the same. The top was clean of bird droppings and anything else visible, but I still cleaned it to be sure that dust and dirt was off. My garage is not well sealed, so it is likely to have dust on the car and on the top. A horsehair brush was recommended as the tool of choice, so I have a US-made bricklayer's horsehair brush: I brush in the direction of the fabric (mine is a silverlight top so there is a very fine silver thread in it, making it look almost grey, but also making it easy to see the fabric direction). After that I have a horsehair vacuum attachment that I use to vacuum anything else out of the top. This way more embedded dirt may get pulled out, dust and other things can be removed easier. Honestly, though this and the brush are ONLY used for the convertible top, it seems that the top looks dirtier (little bits of stuff, not dust) after vacuum. It may be pulling up/out other bits in the fabric, or maybe just shuffling them around a bit so that they are more visible After vacuuming I brush again and then are ready to go. I use BMW top protectant, not because it is a BMW product, but because I like that I can control how much I make, how I apply it, and how anything spreads. Im not a fan of the sprays because of the potential for overspray, aerosolization of the spray, etc. The product gets mixed in a 10:1 ratio with water. There is enough for 2.5L of solution, but youll never use that much on a car. 500mL is more than enough by far. To apply, I utilize a grout sponge. These sponges are very popular in detailing circles now as compared to other methods like mitts and other sponges. The recommendation seemed to be some loose Greece-made sponges found at HD, but I wasnt able to find them. I was able to find German-made sponges though that seemed nice and soft. I cut it smaller for more control. And then wet it in a cup filled with 500mL of solution. The sponge should be well wet, but rung out to not have too much excess. Then the solution can be worked over the surface. Good indirect light is helpful to be able to see what is wet and what is not. I like to work one section (between bars or seams) at a time, both sides, to get it complete. I pay special attention to areas where water will sit, like water gutter channels and for the seams where there is more potential for entry. Just keep applying until the whole top is visibly damp and have had product applied. Then take outside to dry in the sun (instructions say to apply in shade and dry in sun). I also use a microfiber towel after to blot the top to remove any excess that is present, so that the top has a consistent look to it and that there isnt extra repellent that will dry and stain or drip off. A few things to note: You end up using about 250mL of solution on a 135i sized top. But you do not want to make just 250mL of solution, because no matter how well you clean the top, there will be some dirt that the sponge lifts out and will end up in the solution. Id like for it to stay in the solution rather than get re-distributed on the top. Also, it is important to apply while having another rag in your hand. While overspray is not a problem here, and the fluid is easily controlled with the grout sponge, there is always the chance of having some dribble off when you work it into the material. My car was really dusty, so I had to be careful wiping it up. Youll get a residue though if you dont. But its an easy job, works fine, and anything to help protect the convertible top is a good thing.
Thanks for the write up. I don't have any convertibles, but I've often thought of doing this on a pair of my Carhartt work pants to make them waterproof for bird hunting. Having worked with this product, do you have a feel for whether applying this to thick canvas pants would be sucessful?
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