When I slush cast would I be able to add multiple layers of resin?
Absolutely! And, actually, you will have to. If you try to do it all in one go, you'll wind up with a giant gob of resin somewhere, and thin spots everywhere else. I used to use smooth-cast 300 for masks, but switched to smooth-cast 65D about a year ago. Kind of depends on what you're going for, but the 65D has just a little bit of flex to it (not
flexible, but has some give), and overall isn't quite as hard. 300 is much harder, but I found it to be somewhat brittle, for masks, because it is so rigid. With 65D, I don't worry so much about what will happen if I drop one.
I'd start by doing 4 layers, and see where that gets you, thickness-wise. You'll have to play with the portions a bit, but with a mask this size, here's about how my process would look:
Layer 1: 1.5 oz of each part A and B, coat the entire mask as even as possible, and then favor one side, as it's about to set up
Layer 2: 1 oz of each part, coat the entire mask as even as possible, and then favor the other side, as it's about to set up
Layer 3: 1 oz of each part, coat the entire mask as even as possible, and then favor the middle (top and bottom), as it's about to set up
Layer 4: .75 oz of each part, coat the entire mask as even as possible, and then favor any obvious thin spot, as it's about to set up
I wouldn't do a layer with more than 1.5 oz of each part (3 oz total), otherwise you'll end up with an overly thick spot.
I always tint my last layer a dark black, so I can be sure of good coverage around the whole mask, and also when I demold it, I can see any spots which were thin, from the front, which helps me decide how to refine my slushing process down the road.
Also, don't let the resin fully cure between layers. As soon as a layer has set up, and no longer oozes, start mixing the next layer.
It's hardly an exact science. Even with masks that I've made around 100 pulls from, the portions I use, and how I do it varies, just due to how things move around, they're never the same. So you just have to adjust as you go, but good to have a general plan of attack. Your process will be slightly more interesting with the ears, but the overall idea is still the same.