The rubbery Sealant might come of if you find a piece of Plastic to use as a Scraper.

If you cannot get at some of the Rust but you can degrease so it is nice and dry you might consider one of those Rust Converter type stuff that come in a Plastic Bottle. AutoZone and other places sell it.

It chemically converts the rust into something else and has a surface that can be painted.
UV light is bad for the Rust Converter after it dries all it needs to be top coated but also since it is under the Trunk it is not going to be exposed to much light anyway.

The one I used is about $8 a Bottle. If you want it to last longer do not dip your Brush/Applicator inside of the Bottle. Pour a little into a Glass, porcelain or enameled Cup and use that; putting the Cap back on the Container as soon as you can.

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84 300D, 82 Volvo 244Gl Diesel