Voxel Sniper help

Hello there, I need help with a little issue I'm having with voxel sniper. I know it has many many different ways it can be used but one thing I can't figure out is if it's possible to make a layer over top of a current layer. The whole build is made out of snow and I used a brush to put a layer of ice over top of the snow in small sections and the outside turned out nice but what I didn't realize was that I was also filling the inside of the tower with ice as well. Basically it didn't layer like I thought it would and ended up filling everything I clicked on all the way to bedrock. Any suggestions on how I can layer just the outside without affecting the inside?
This is the outside
This is the inside
Sorry if the pics look cutoff but if you can tell the inside is filled with ice minus what I got rid of. Anyways if anyone has any suggestions or answers that could help out that would be wonderful and thanks!