Stop. Do not pass GO. Go directly to the PPFA bookstore (or any other vendor that carries it) and buy Chris Pashke's book on mounting.
http://services.pmai.org/online/core/orders/product.aspx?catid=7&prodid=318
Seriously, how can you own a piece of equipment and not know everything about it before you try it (especially on customer's art)?
Since it is "used" you need to be wary of a few things:
PLEASE invest in some temperature test strips to confirm that your thermostat is accurate. How can you be sure that you are using the proper temperature if you have not confirmed that the dial is calibrated? That you have all of the heating coils working and that there are not cold spots on the platten?
Please check the condition of the sponge pad. It is the usually the first thing to wear out and you will get uneven pressure (usually the edges will bond first as the center wears out). A symptom of this is bubbles in the middle of the piece.
What are you mounting to? 3/16" fome core is thicker than s or x board. Gator is more resilient than foam, even though it is the same thickness. You need to confirm that the pressure adjustment is set properly and that the 1/4" masonite "laminating" board has not been "stored" under the foam pad.
Come to the WCAF or the PPFA Convention and take a mounting class! There are many variables as a prior post mentioned- what you are mounting, what you are mounting to, what you are mounting with!