What specifically is wrong with the tension? A 12 wt is even bigger than a 28 wt so if you could make that work you should be able to make the 28 work. The main thing is getting the needle size and type right for that size of thread. You'll need a 90/14 topstitch needle at least and maybe a 100/16 topstitch needle for the biggest threads. The topstitch needles have a bigger eye and handle the bigger threads easier.