TPO = Thermal Plastic Olefin.
It's always nice to know what you are dealing with. Sadly, TPO doesn't tell you much as it is a blend of plastics and *other*.
Thermoplastic olefin - Wikipedia, the free encyclopedia
In my current build, the body panels are polypropylene (PP) and nothing will stick to the stuff easily. My BMW had ABS, which is much easier to deal with. Polyethylene is also extremely challenging; in fact it is used for vacuum bagging films for that reason.
But there are things you can do. As others have stated, polyester resins are useless. You can drill holes, and when the polyester cures, it will mechanically lock into place. But you will always have delamination issues, and that can be problematic because to pod will resonate against the pillar trim, and you get air gaps, looks like ****, ect.
One way to get around this is to mold the pods to the pillar, but make the pods as separate parts that can then be mechanically fastened to the pillar. You can use polyester resin for this, just wrap the pillars with foil or release wax/film so you can separate the pods easily. And then figure out a fastener scheme.
Another approach would be to use epoxy. If you are lucky the epoxy may stick to the TPO. I have never dealt with TPO, so I don't know for sure. But if the TPO has a high composition of PP or PE, you will still have problems. With that said, I have used epoxy with my PP trim panels. The epoxy does not stick, but if you rough up the surface with 40 grit sand paper, the epoxy will mechanically lock in place. The epoxy is way stronger than polyester resin, so it should hold well. But, I am willing to bet I could delaminate the fiber glass I laid up over the PP if I were to really stress the part.
For prep, I first wiped the part down with acetone (test first to make sure it won't damage the TPO) to remove any parting waxes, cleaners, whatever. I do this first so that when I go to sand, I don't end up spreading around and grinding in any contaminates. Then I sand, then I go back and wipe with acetone.
The biggest draw backs to epoxy is the cure time and expense. You cannot use regular body filler with epoxy,
you must use epoxy filler. This can easily mean 6-12 hours cure time between filling and sanding steps, not 10 minutes like typical polyester based fillers. You can make your own filler by mixing in fairing compound (which is the easiest filler to sand), but there are some pre-mixed epoxy filler also. I would not mix and match epoxy brands or types though. Be mindful of exotherm when mixing the filler. For high strength bonds, epoxy with colloidal silica works the best, but it is a beast to sand...I mean a real pain in the ass, so use it sparingly.
