Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
The key on poly shirts is to reduce the pressure, there is no other options.Given the heat @300 degrees and anything like medium pressure you are going to semi melt the poly fibers and flatten the thread profile. That is where the shine and silvery look comes from.Try putting a pillow under the shirt we use a stack of sweatshirt material about 3/8 to 1/2 in thick and press at barely zero pressure.The temperature at 300 is sufficient to soften the fibers and when over squeezed will flatten the fibers to some degree. That is where the visible shine comes from degrading the shape of the threads that are 100% PLASTIC. TEST TEST TEST but this is what we have found you need to press as lightly as possible and the pillow helps distribute the what pressure there is evenly over the image.mooseman