What size rubber grommet will a 3/4" PVC pipe fit (water tight) through?
In these situations, I use bulkhead fittings. It saves the annoyance to getting this stuff to seal properly, any slight leverage and there's now a leak. These are available in several sizes at most plumbing supply stores. If there's a gentle curve to the surface this has to penetrate, I've used a large O-ring instead of the supplied gasket. They're available as glue type fittings, or internally threaded.