Residue can accumulate in the top of the drain above the trap, esp. where the overflow meets with the drain. Occasionally, I pour a tiny splash of bleach down the drain, then let it sit for a bit. That takes care of the smell for a couple months, then we repeat. Another method would be to remove the pop-up, then scrub the drain walls with a bottle brush...but that can be messy if you remove the pop-up lever to do it,

I've had a bad smell coming from my sink. Last fall I pured Liquid Plumber in, let it set and flushed it down. I poured vinegar (straight) in, let it set and flushed it down. I used bleach and flushed it down. Each time, after a day the smell came back. We left for 3 months this winter and when we came back there was no smell. Now,after a week it started again; I poured Simple Green in the sink-straight so it would set in the goose neck pipe underneath; then flushed with HOT water in about an hour. Smell went away, then 2 days later it is back again. Now...I am thinking that because it's an earthy/moldy smell,, could it be a broken pipe underneath and the insulation is wet and rotten ... could that be it? We also remodeled that bathroom 4 years ago and had a new sink put in, but not new plumbing. HELP PLEASE! We use Mentadent toothpaste, but rinse it well with warm water afterwards. Our other bathroom has never been remodeled and hubby uses it every day and it doesn't smell at all.

It may be your vent, or could be your sink drain. I found out my problem/cure from googling problem. We had recently replaced the sink stopper and faucet in bathroom. Shortly afterwards noticed sewer smell occasionally, few times a day. Yes, sewer smell is a skunky smell most times. My googling efforts said to check the overflow of the bathroom sink to see if clogged. I checked by using a funnel and pouring water into it to see if water drains. Sure enough it was clogged. When installing drain stopper we got excess plumbers putty into the hole that drains into the pipe. We are amateurs at home improvement and something usually goes wrong/not smoothly. After some fiddling with something to clean out putty and water drained properly no longer had sewer gas. It was explained on the Google info the clogged overflow sometimes causes some weird suction when toilet is flushed causing sewer gas to come up... Good luck.

I'm not sure if this is comparable?...Our house had pine paneling in most of the rooms. Stained and varnished several times over the decades. 50+ years of smokers in the house made our house smell like a dirty ashtray. I scrubbed the walls so many times over the years and still smelled of cig smoke/ash tray. When I painted most of the paneling, I primed twice, then two coats of high quality paint in a semi-gloss (more sealing that flat). That helped very much. The bedroom and coat closet that did not get painted still smell of cigs/ash tray, so I know I'm not imagining the improvement. To me, the cig smell is similar to cat urine, in that it's a forever scent once it's built up and very hard to remove. But the priming and painting sealed the cig smell. Maybe think of painting the cabs after a primer? Not sure if applying the schellac like you just did would seal the smell in. Maybe as it cures, it might?