It'll be easy. One tap with the applicator, and it overwhelms their senses with a sort of "feel-bad" sensation. Then they're gone before you know it! You could test it out on just a few Mole Ratties, but for real testing, try it on ten or more. There should be plenty in the Tepid Sewers, downtown.
The sewer has a straightforward layout, connecting an extensive sewer on the Potomac end and a collapsed metro station on the Georgetown terminus. There are plenty of supply caches scattered in the tunnels, filled with molerats and raiders hunting them.
The metro tunnel connecting the sewer to the Georgetown network and water system also houses Rocksalt's base of operations.
Dean's Electronics when you first enter the subway area from the south. Deal with 3 raiders (1 behind sandbags, other 2 in corridor), enter the corridor, turn left, and enter the small room on your left. The book will be on the right corner behind toolbox (near ‘generator’) (A).
Be careful when coming up to the raider's hideout. There are several mines scattered on the tracks, so either shoot them and possibly cause damage to one of the raiders or disarm them and keep them for yourself.
There is a safe with some worthwhile loot amongst the raiders (B). You must either loot Rocksalt's key off Rocksalt(C), or pick the lock to open it. It is in the sleeping room of the raiders. Rocksalt's key also opens up a storage room further on in the area. There is also a pre-War book on top of the bookcase.
There is a power fist in the terminal storage room, in a metal box (D). In a wooden crate with some burned books is also another pre-War book.
There is a workbench in the top right room on the local map, along with tools, scrap metal, and junk.
Xbox 360 There is a rare bug that when you unlock the door using Rocksalt's key and open the door there will still be a barrier like the door was never opened in the first place. The only way to fix is reload a previous save.
Another glitch is when Rocksalt's raiders kill the mole rats, a spurt of blood may appear where they were killed and continues to shoot blood. Found in the room with the generator.
PlayStation 3Xbox 360 If you complete the quest then steal the Repellent Stick (hidden upstairs in Moira's shop) it will be irremovable as it still counts as a quest item but will not weigh anything. You can use it as a weapon but it cannot be dropped and will not appear in your inventory when checking in a container or a shop.
Xbox 360 After entering the Tepid Sewers an Enclave deathclaw will immediately spawn in front of you, then die in a matter of seconds. This is possibly somehow connected with the deathclaw control scrambler which basically turns Enclave deathclaws into short-term followers.
Xbox 360 Once you go into the sewer it's possible you won't be able to get back out due to a door lock glitch.
↑The Lone Wanderer: "I'll test out the Repellent for you." Moira Brown: "It'll be easy. One tap with the applicator, and it overwhelms their senses with a sort of "feel-bad" sensation. Then they're gone before you know it! You could test it out on just a few Mole Ratties, but for real testing, try it on ten or more. There should be plenty in the Tepid Sewers, downtown." (Moira Brown's dialogue)