I thin my Elmers down - about 3 parts glue to 1 part water using an extra glue container and apply in a thin line to a manageable section of the batting - i then "spread" out the glue gently using an old credit card. I found that applying it to the batting then smoothing the fabric down caused less "stretching" during the process. Next step - i press that section with a warm iron to speed up the drying process. This method has been working very well for me.
RE: gumming up the machine..... i always let my sandwich dry overnight before i start quilting.