If you've got, or can borrow, a small compressor then buy yourself a cheap underseal gun and use the proper stuff. It's much cheaper, easier to apply, thicker(because it's forced through a much bigger nozzle), gives a better finish, is available in different colours or overpaintable etc,etc.
My gun was about £5, and the underseal is about the same per can. My local paint supplier often has out of date products for pennies on the pound, I always buy underseal and polyurethane sealants when I see them. It's rare for me to use a whole tube of Sikaflex(or whatever brand) in one go, throwing away a half used tube hurts a lot less when it cost £2 and not £8.
If spraying thick stuff from anysort of can, stand it in hot water for about ten minutes before using it. And they're not joking about the time you should shake it!