Oh, I totally dropped the ball on this one. Thanks for the clarification. The above information is still true for if you want to seal a hole as they will all provide a waterproof seal as long as you apply it correctly and choose a non-shrink product. However, you’re trying to seal the entire nest, which is a different process I’m not as well familiar with. If I need to seal a large surface, I roll on some primer and topcoat, using marine grade paints if it needs to be especially weather-resistant. Not too expensive, but you need to buy a lot at a time – not to mention this is a much smaller area – so probably not the best fit for you.
If I understand correctly, firebrick is highly porous – you’d have a tough time keeping it hydrated, especially if you are heating the nest. Regular paint will prevent the material from breathing but I don’t recommend you do this since the paint will degrade if kept in wet conditions. Paints used for marine applications such as epoxy polyurethane will work, albeit not the best fit for this case.
The rubber sealant is probably the waterproofing tar material. Liquid rubber, they call it. That stuff is nasty and I stained a bunch of my clothes with it so I’m not a huge fan. But, it’ll work. Absolutely no need to worry about VOCs. Just leave it outside for 48 hours or put it in front of a fan. Make sure you apply it outside on some cardboard and wear clothes you don’t care about. Use a disposable paint brush. This is if you buy the gallon version.
Aerosol version: I don’t have experience with the small aerosol cans (I assume this one?
https://www.homedepo...R20/203144520),but I imagine it’s much cleaner. I still advise spraying it outside on cardboard. Don’t touch when wet! Vinegar will get it off if you do though.
One way to circumvent this problem (long-term hydration and ease of maintenance) is to approach it at a different angle: Have you considered adding some sort of water reservoir that can be easily refilled to the nest? You don’t even need a sponge, just the water tank needs to be able to evaporate – yet covered or hidden so ants don’t drown – to allow the water molecules to reach equilibrium in terms of evaporation and condensation (this is humidity). Reducing airflow into the entire system will reduce water loss. Customizing the nest as to surround the firebrick with a non-breathing material from the get-go removes this problem entirely. In certain situations, modifying ambient humidity will reduce evaporation rate. I don’t have any idea what your setup looks like and providing a picture can help generate more ideas.
In short, the aerosol liquid rubber spray seems like the quick and dirty solution for your situation. I’d recommend that.