How are the salt buildups flushed out of the planters in ebb & flow?

When you are doing your routine daily topping off procedure see reservoir maintenance, you will be adding just plain ph'd water on every other topping off day.
Pour the water into the system by carefully pouring it thru the planters. This will flush the salts out of the rocks and into the main reservoir.

Also, on the days when you are replacing the reservoir solution, it's a good practice to flush some straight ph'd water thru the planters, allowing them to drain off before you refill the reservoir.

Another method, if the clones have'nt yet grown their root mat outside of the planters, is to simply dip the planters in fresh ph'd water a few times, letting the water drain out carrying away the salts as they drain.