The first time I ever used Iron-X, I had noticed that I was going through quite a lot of product and was quickly running out of a 500ml bottle on my SUV. Having never used the product before, I did not really know what to expect?
Other's mileage might vary, and no doubt for a smaller car, there's not quite as much area to cover, and thus a 500ml bottle I think should be adequate, to even do the wheels too as well.
I might imagine this question is being asked for perhaps a couple of reasons. First its cost, and to be a bit more thrifty and stretch the product, and/or not be wasteful.
I'm not sure where I got this idea from, perhaps from another user, or the carpro forum, but what I did, was had a small bucket of clean warm water, and a small soft kitchen sponge.
I wet the sponge and squeezed out excess water, and instead of trying to coat every little last square inch of paint with the product, I gave each panel or section a few healthy squirts, then spread the product with the dampened sponge.
I think this helped two-fold, in that I was able to use slightly less product, and it also seemed to help with getting even coverage, and that the product was not drying so quickly after application. As I went around the vehicle, I periodically rinsed and again squeezed out the sponge and continued on.
The main application trick is to apply quickly, but completely, then step back for a few minutes and let the product work. Best to choose a calm day, no wind, and definitely in the shade.
Whether my methods are orthodox, and acceptable, I am not entirely sure, but it seemed that my method didn't adversely diminish the product's effectiveness.
For those with large vehicles, or perhaps wishing to treat 2 smaller cars, I'd probably advise to at least spring for a liter bottle.