I have eight ProTanks, plus several Kanger T3s; here's what I've found:
1. The "rubber" taste is from the bottom grommet (the one that pinches the wires) burning or scorching. This is caused by vaping the PTs (or T3s) at too high a wattage. These are definitely low-wattage devices. I vape mine in the 7-8 Watt range; any higher and the burnt rubber taste starts coming out.
2. Almost all the stock coils are wrapped WAY too tight; chokes the wick and restricts juice flow ... and efforts to improve juice flow by mucking around with the silicon cap that seals the tube only promotes leaks. There is no way to correct this other than learning to rebuild the heads; I rebuild all my PT and T3 heads with cotton cheesecloth and I have ZERO dry hits and ZERO leaking.
3. If you elect to learn to rebuild your heads (and I really recommend it; it's not as hard as people fear, and the results are well worth it!), I also recommend getting a wire welder (I use the Spotty v1; got it from the list on Vapor Wall, and it's excellent) and rebuilding your heads using NR/R/NR wires. This will completely eliminate the "hot legs" issue, which is the biggest contributing factor to the burnt rubber taste. I've also had success with the twisted legs solution, which ameliorates the burnt rubber issue, but the downside of this method is leaking ... the twisted wires are fatter than a single strand of NR wire, and the seal of the bottom grommet (the wire-pinching one) is not as secure as with the single NR wire.
4. As a bonus, re-wicking the ProTanks (and T3s) with cotton instead of silica really improves the flavor! I vape some pretty intense flavors (e.g., Boba's Bounty, Grant's Vanilla Custard, etc.) and I get just as full a flavor from the rebuilt PTs as I do from my high-end attys. Cotton wicks match the flavor of genesis atomizers in my experience ...
5. I wash my ProTanks in hot, soapy water in an ultrasonic cleaner; I use a generous squirt of Dawn dishwashing liquid (the best grease cutter I've found!) and distilled water. Two eight-minute cycles in the USC, and then a rinse in clean distilled water, removes all the flotsam and jetsam from the manufacturing process ... the amount of dirt and metal shavings that come off in the USC is simply amazing. No wonder so many of these taste rotten out of the box!
6. One further hint: if you have an ultrasonic cleaner, drop the heads in a shot glass full of pure grain alcohol (PGA; I use EverClear 190 proof from my local liquor store) and set the shot glass in the USC with warm water up to the level of the alcohol. A couple of cycles, and the heads are good as new! No USC? Just an hour-long soak in the PGA works about as well ...
Aside from my high-end atomizers (Penelopes, Ithakas, Russian 91%s, etc.), the ProTanks are the best toppers I've used
after I stopped using the stock coils. As always, YMMV
