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How would you seal this gap around kitchen sink plumbing?
(sh.itjust.works)
This is for a rental unit, so I'm trying to keep the cost low, while also sealing it away from silverfish. I have very fine steel wool on hand as well as a tube of DAP ultra clear flexible all purpose sealant.
Assuming you can't just deal with the pest problem because you don't control what's on the other side of the wall, I'd put in steel wool to keep rodents out, and spray foam this side of it to seal off the insects.
Home Depot sells a high-expanding foam that's supposedly pest proof. The Great Stuff Pest Block is probably what you want.
I used the pest block one (green can) to seal a few gaps in my house and it worked great! Keeps bugs and weather out!