This primer is a acrylic primer. That's what the polyurethane means. This means that it's not very durable as it's grip on the plastic is weaker. Acrylic paint will not damage the primer and neither will enamel paints. Lacquers are a bit trickier as too much will melt the primer (and possibly the plastic underneath) and cause the paint to become splotchy.
However, if you bump a key or something hard onto the bare primer, it will scratch. It is not very durable at all. This issue is reduced when paint is applied as the top coats will harden and help protect the kit. If you try to sand this primer, then you'll probably fail. Because it gets damaged so easily, the sandpaper will only do that more.
The white primer like all white primers take lots of coats to become white. However, a light coat of grey primer under the white primer helps out a lot.
Just go with a Tamiya spray can primer.
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