Quoted from Nokoro:Where would one buy this, and are there any brands or types to get? I asked at Lowe's but they only had stuff for drywall and didn't think it would help.
Look for it at a marine/boat repair store or an auto parts store.
Something similar to this:
https://www.amazon.com/3M-Fiberglass-Resin-Repair-Pint/dp/B000BO9NOO