I find the ballooning of the black Rutland's (as I used to apply it, and as most still apply it) a tiny bit irritating, as it absolutely does change the hamon. It will follow the 'line'...but the effects above and below that line will be different. The 'habuchi' for example, will not be the same as if the clay were dried properly. That said, you shouldn't be using a thick enough layer to balloon much, and if you dry that thin layer in a tempering oven for 30-60 minutes, you'll find the puffing up of the refractory to be pretty much eliminated.