N00b question...

Ok so I understand that metal stores accumulate via dispensers, but what happens if you need to get a sentry down fast?  Are you just left with 70 metal until you destroy the sentry and then build a dispenser?  Am I just screwed as a defensive engineer on no-setup maps in terms of being able to stop rushing Scouts?

you can pick up ammo crates (the spinning boxes that give you ammunition) to refill your metal gauge the amount of metal differs from the size of the box

or you can pick up a dropped weapon from a dead player and you will recieve metal.

during setup time it is good to remember where the ammo crates are so that once you drop a building you run to get more metal then drop another. such as dropping a dispenser rushing to an ammo drop then dropping a sentry

@Nooch: some times you have no choice but to drop a lvl1 sentry and leave it to try and find a better location for your setup.  I'm not the best engineer, but I've learned that unless you are playing strictly defensivly, then you should find a good spot for a lvl1 sg and leave.  You should have found a sutible spot before it reaches half ammo.

Well for one, you as an engineer aren't supposed to be the only guy who takes care of rushing scouts, if your team has any sense of teamwork at all But if you do have enough metal for a level 1 sentry, just build one and go for offensive engineer for a bit. Grab some metals from ammo boxes or weapons, then go back to SG wrenching. One thing I like about being an engineer is that it will for sure forces you to improve your shotgun aiming, as unlike some other classes who also has shot guns as their sub weapon, its a primary weapon for the engineer. As a casual gaming engineer, I would say that getting metal from sources other than your dispenser, even in a slightly critical situation, is not that hard. Unless you are in competitive play