Here is how you can use a heat gun ( preferred) or a hair dryer ( works but slower to heat) to heat and soften the urethane so that you can CAREFULLY cut into your mask with an X-acto knife. Please please, cut slowly and away from yourself!
Be very aware that there are thinner and thicker spots in the mask you will encounter during a cut. Going from a thick section to a thin section may cause your knife to SUDDENLY accelerate and cut a finger which is exactly how i just ended up with five stitches in my left thumb. It sucks. Wear a cut resistant glove and be patient!
Concentrate heat on the BACK of the mask more than the front. There are little air bubbles in the urethane thatmight present as litttle domes on the surface after heating and expanding. If you get one of these on the surface of your mask just cut it open with an xacto and fill with putty. I use Testor's contour putty for small holes and apoxie putty for larger ones. Fill, and then sand flat once cured.
Also if you are cutting a jaw piece off and want to keep the shapes pretty flush to each other once the jaw is closed, fill any uneven spots with Testor's contour putty rather than sanding them out too much. You may find little lines in the cut areas like the ones im pointing to with a needle here:
If you try to get rid of them by sanding or dremeling them away, your pieces won't lines up as evenly! The putty will fill it and then can be sanded flat once cured.
UPDATE! Here is some progress of me prepping and reinforcing an owl mask who's jaw gets cut away and hinged:
BEFORE CUTTING! The light putty is Free form Air, the darker putty is 5 minute plumber's epoxy. I leave an open part where the jaw seam is going to be. This helps the whole mask have a reinforced shape before you cut anything away.
Trimmed , sanded and jaw has been cut. I then stick the jaw back together with 2 beads of super glue. Not too much glue, just enough to hold everything steady while I glue in the hinge:
Once the hinge is glued in, i use super glue remover ( pretty much acetone) to free the jaw again. Be careful, acetone can melt certain plastics! The clamps were there to hold the hinges on while the glue cooled. I use high heat/high strength Steinel hot glue for now. Once the mask is finished the hinges will get reinforced onto the mask with plumbers epoxy.
open and ready!
You can see how uneven the edge of the mouth is since the mask thickness is not perfectly even. I use apoxie clay to backfill this!
Apoxie clay applied! I use water and a brush or gloved finger to smooth it down. Once it is cured overnight, i can sand it smooth and add more apoxie if needed until the mouth ridge is even and closes nicely!
Apoxie clay applied! I use water and a brush or gloved finger to smooth it down. Once it is cured overnight, i can sand it smooth and add more apoxie if needed until the mouth ridge is even and closes nicely!