Hi Dabbler
When I first started experimenting with the vapourisation of Oxalic Acid, crust-formation frequently occurred when trialling a small cross-sectional area pan, but not when using a larger pan. I concluded that crust-formation was being caused by the material 'bubbling', such that the 'bubble skin' became suspended above the heat of the pan by it being attached and thus supported by the slightly cooler sides. With the larger area pan, the sides were sufficiently far apart to not support a 'bubble' in this way, so that any formed simply collapsed back down to be further heated and thus vapourised.
With an existing applicator - assuming that it's satisfactory in every other way - what I would suggest is that you weigh the crust which is formed and left behind. I'm fairly confident that this will be a small amount. If so, then simply increase the first dose by that amount, then add the normal 'dose' (I use one gram per brood box) to each subsequent application, along with any crust remaining from the previous treatment. Then, at the end of the day, simply dispose of that last remaining crust.
'best, LJ