I really thought you could activate Sunder multiple times, so that with enough advantage, a lightsaber could cut right through a blaster or whatever.

You can, but you can not activate it on different targets, so basically you have to stick to one enemy blaster and cut off both blaster barrels in one attack. Makes me a little sad, but I can understand the idea behind it.

I really thought you could activate Sunder multiple times, so that with enough advantage, a lightsaber could cut right through a blaster or whatever.

Sunder is called out as one those that can be triggered multiple times in its description, thus allowing for things like a lightsaber slicing an enemy's weapon in half. But it's generally restricted to a single hit; you'd need to have enough advantage to trigger Sunder on separate hits, especially if you wanted to use Sunder on the weapons of multiple adversaries. Then again, if you're attacking a minion group, the GM may allow for your usage of Sunder to impact all the weapons of that minion group, such as using a lightsaber to cleave the barrels off the rifles of an entire stormtrooper minion group, but that's more of GM fiat than anything really addressed in the rules.

One "fuzzy" point for some is where a Triumph comes into play when activating something like Sunder or Knockdown, both of which can be activated multiple times, though with Knockdown you only need to activate it multiple times to affect a target that's above Silhouette 1 (so 3 Advantage to knock a Silhouette 3 target on its arse). Technically, a Triumph would only count as a single activation of those talents, which can understandably be seen as a waste of a Triumph in comparison to "sexier" options like upgrading checks or triggering a crit with a weapon that has a high crit rating.

From past discussions, some GMs have allowed a Triumph to count as multiple activations of Knockdown and Sunder. With Knockdown at least, it makes sense to allow that, as your attack generates the extra oomph needed to take a bigger target down. With Sunder, it'd essentially mean that the PC gets to automatically destroy the target's weapon, which may seem a bit much for a GM, especially if it's a combat-focused adversary that is dependent on a weapon that's not immune to Sunder in order to be a significant threat to the party.

Doesn't it take 4 activations of sunder to destroy a fully functioning weapon anyway, so normally 4 Advantage? I can see a house rule of Triumph being worth 2 Activations of Sunder being quiet a good rule. Certainly destroying a Nemesis weapon in one attack is bad news, but that's when the GM flips a DP to have a backup weapon ready to go.