Underseal is terrible if applied to a wet rusty underside as Kipper says.
The same as painting damp wood!
I had my EJ in a garage for a week with a dehumidifier before starting to treat the couple of rusty areas.
My somewhat rusty car has no rust whatsoever where you've shown - so dare I ask what its like around the rear sills underneath and inside the front arches (under the top of arch liners)?
As hinted above there is no "rust free" example unless its been kept in a climate controlled garage from new and only driven on dry summer days. They rot from the inside out and people only seem to care about the rear arches (which are mostly cosmetic!).
People also seem to avoid posting pictures of the rot - I guess in case it affects the future value of the car. I'd rather see the true state of it.
These pictures are of a red b18c4 converted EK3 for sale on FB at the moment. Its the worst I've seen but I'm sure there are many worse!
The photos were posted of a previous owner who sold it to break....
The first picture is where a torque mount should be!