G'day slashnburn,
The acetone performs two functions in the mix.
One is, as you say, to 'thin' the mix [i.e. reduce the viscosity] to make it penetrate better.
The other is to allow the mix to 'wet' metal surfaces that are already wetted with water. That's where 'polarity' comes into play.
You know how oil will just roll off any water-wet surface? The acetone is there to prevent that. Straight hydrocarbons, like kero, won't do it.
The MIBK [Methyl Isobutyl Ketone] in PVC primer [the glue is thickened with PVC, so it's not what you need here] is a close chemical relation to acetone [aka Dimethyl Ketone], but it's much less volatile [Boiling Point 118 �C as against 56 �C], though not as good for dewatering. So it can work more better in hot climates, as it doesn't evaporate before it can do its job.
Just as an aside, I use a home-brew solvent mix [called 'Ed's Red'; an update of a US Army Frankford Arsenal formula from the 1920's] for firearm bore cleaning, that's equal volumes ATF, Acetone, Mineral Turps and Kero. Still penetrates quite well, and shifts crud very well. At a lot lower cost than the commercial products.
