When you sand and polish the metal parts that sandwiched leather pieces, these metal layers get hot and transmit that heat to the leather strips, which gets contracted, resulting in an end piece with the metal and leather parts at a different level. You can feel and see the difference, and it's not the proper finish I'd like my customers to get on their knives.
I've been doing this for over a year now, with excellent results.
Strip of leather (8" x 1 1/4" x 3/16"), plastic bag to contain the leather and resin and about 50 grams or prepared polyester resin. This ammount will be more than enough to impregnate this piece. (Picture 1)
Leather strip and resin already inside the plastic bag (if anyone's wondering, bag thickness is 60 microns) (Picture 2)
After pressing the bag to get rid of the extra air inside, I tie the bag to close it. (Picture 3)
Getting the bag inside the pressure chamber. (Picture 4)
You can read more about it on this Tutorial: Polyester Resin Impregnation II
Closing it. (Picture 5)
This time I use pressured air. This means that I put 120lb/inch for 15 minutes to the hermetically closed tube / pressure chamber. (Pictures 6 and 7)
Once I get the bag from the pressure chamber, I open it to get the leather strip out (resin doesn't get glued to the polyethylene bag) (Picture 8)