Home Depot sells Flex Seal spray. I don't know if they have the brush on or not. I have only used it on a Motor Home Roof Leak so I don't know what it would do on the Boots. You perhaps need to read up on it. They have Black.

Get the rear wheels off of the ground and after cleaning get the axle you want to spray rotating slowly and spray it as evenly as you can. I don't know if one can would do all 4 boots but do the worst looking boots first.

If you are willing to experiment.

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84 300D, 82 Volvo 244Gl Diesel