Getting sheen is decent on him if you want to trade harder. It may be true that singed doesn't have many ways to trigger the passive but singed can keep someone tied up for 10-15 second during an average trade between flip and puddle. This gives you a lot of time for you cd's to come back up. I don't know if I would go all the way with lichbane though. You won't be auto attacking much during the late game. You'll mostly want to replace the sheen with items like force of nature or rylais that will allow you to give the enemy team highfives more often.
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