I left a message earlier and I did pull the trap and wash it out. There is still a strong bad smell. A plumber that was here 1 year ago and working on the bathtub drain pipes, had me clean out those pipes as they came loose. There was ugly smelly goop in those pipes. The plumber said it was old drain cleaner that got stuck there. When I cleaned out the bath pipes it look like a gel and made me gag due to the smell. We have only 1 bath so I know the pipes from the bathtub and kitchen sink to the down drain are clean. So this stuff must be in the pipe line between the sink and where it hooks up to the bathtub and sink.

What could I put down the sink that might eat through this stuff? I thought about coca-cola as it can eat rust off screws.....any other suggestions?

Use baking soda, drain cleaners are an acid, the baking soda will nutralize it.

There's a good chance the smell may be coming from the overflow channel, and not the drain itself.

Hair and soap scum (which is made of vegetable oils) and skin cells end up in that overflow channel and can start to rot in there.

Take a whiff of your overflow drain which you should find on a bathroom sink. If you figure that's the source of the smell, then I'd liberate an old car speedometer cable from your local auto wrecker's, bend the end of it, put it in a cordless drill and use that as a poor man's snake to clear your overflow drain.

The way I do it is by taking the P-trap off and putting a special ABS fitting I made up on the sink drain tail piece so that water can't drain out of the sink. Then I fill the sink up so that even the overflow channel is full, and plunger the sink. The surge of water can't go down the drain, so it goes UP the overflow channel spewing all the rotting putrifaction in there into the sink. But, don't do this unless you've got a PLASTIC p-trap and plastic hub-trap adapter under your sink. If you have a chrome plated p-trap, don't take it apart because you'll never get it back on without it leaking. Then, you'll have to start replacing the p-trap, and that can be a big job.