Teax is a solvent, similar to Testor's bottle cement. It has no inherant filling properties, but if you press the partially soften seams together, you may force a bead of molten plastic up which can subsequently be sanded. It cannot be used for PE or resin, only for regular plastic to plastic welds. It is very volatile, leaving the bottle open overnight will allow much evaporation. I used a technical pen/drafting pen to apply it. (I'm told they're now pretty much antiques--like me!) They are similar to the pen at the bottom of this set.
It's best to place the parts together and run let capillary action pull the Tenax along. That or apply it to the mating surfaces of the two parts and press them together. Curing time is somewhat longer than CA, thus it's very good for indy track links.