I like and use SPI clears and to me the Universal is the softest
by far. that's why it can be buffed so easily, even weeks later.
If you want a hard clear, try their Euro.
I use that when painting dark colors like black because
It makes it so much easier to rid the swirls, because it's a harder clear.
(It's much cheaper too)
I like the Universal for my plastics, like bumpers, just for the
reason of being softer and more flexible.
If you want a clear that gets as hard as concrete in a couple of
days, try the Nasson by DuPont.
It's the hardest stuff I've ever seen.