An internet search for Diesel Fuel Tank Sealer will likely bring up more then on product.

I would think that for all of them you would need to remove and clean the Tank according to the Sealer Makers instructions.

For sure the Tank Gauge Sending Unit needs to be removed.

The POR15 Website would likely have more detailed info on the subject.

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84 300D, 82 Volvo 244Gl Diesel