Yus white spirit is ok and better than warming it up. If you don't have an underseal / Schutz gun then you can get away with an ordinary spray gun too. Machine Mart and the like sell sub twenty quid underseal guns that include an extension probe for getting into all those internal nooks and crannies. The DIY hand applicators that they sell for Waxoyl are something of a joke but if you don't have a compressor then you are somewhat limited.
Oh, and don't forget a plastic sheet to catch all the drips