It gets hot as hell on stage, and as long as you're staying hydrated that means you're going to sweat your ass off. Though maybe not as much as Jason Newsted did back when he played with Metallica in the '90s. That dude was sweating so much he was actually breaking basses on tour.
In an interview with Bass Player, Newsted talked about his insane testing method for basses after he got tired of breaking so many thanks to excessive sweat. The testing method involved a bathtub of salt water, a coil heater, and a stand – one bass survived.
"For Load, I used a '58 P-Bass and a 1981 Spector NS 4-string. The Spectors were built really well, played well, and sounded excellent, but I had a lot of trouble with sweat getting inside. All of my instruments had to be salt-water proofed. We'd done an outside show on the last tour, and by the end of the gig, there was one functioning bass out of six – bad news. And when a bass going through 250,000 watts of PA all of a sudden shorts-out, people are not happy."
"When I was trying to figure out who was going to build my basses for the tour, I'd have to test each bass. I'd fill up a big tub with super hot water, dump in some salt, and submerge the bass. Then, I'd take it out of the tub and put it in front of a coil heater for a few minutes. I'd repeat this same process three times with each bass. Then I'd beat the crap out of it for a while in my studio, and finally I'd let it sit on a stand for a couple of days. Usually, the bass corrodes and doesn't play anymore. None of the basses passed that test except for the Sadowsky."