Kapton works well. Although we have to admit we don't spend a huge amount of time getting it bubble free, this goal has always led to madness. We haven't tried the washing up liquid method yet though.

We just finished printing on Packing tape, for polypropylene. First we tried Tufnol and PVA but to no avail. Packing tape (made from PP) worked fantastic.

PP_6cmPlaque_sampler.jpg