Experience with flex-seal or other spray-on-rubber-sealant

We have a leaky downstairs sink. The leak is right in the drain itself, if you look under the sink it’s leaking out around the grout, above the U-bend.

I was thinking this might be perfect for “flex-seal”, or one of those other spray-on-rubber-sealants. After reading some reviews, some people are skeptical… but they seem to be trying to fix MUCH more difficult things than my sink, or complaining about things like the appearance or having to re-paint, which are non-issues here. In fact, this seems like the perfect use case, in that it’s awkward to get at (so just being able to spray from a spray can would be a life-saver), but there’s very little water pressure (because it’s just drainage from a sink).

A $10 can of sealant vs a visit from a professional plumber seems like a big win, if it will get the job done.

Anyone have any guidance?


Flex-seal also comes in little cans that paint on, if you can’t dismount the entire sink or get at it easily with spray can, I’d say use the paint on stuff instead. It does work pretty well and if it doesn’t work you’re no worse off than you are right now–put a bowl underneath so you can tell if it starts leaking again–if it does, THEN you call the plumber.

Jesus H Christ. My first glance at your thread-title read:

Experience with flea-sex


I have used this stuff with remarkable success in similar situations. Get under the sink with the damp cloth. Wipe off any dust or flaking paint or rust that you can get too.

Don’t go nuts with a scraper or file just wipe off the loose stuff. Let it air out really thoroughly if you can.

Take a deep breath and get in there and spray that stuff all over the place. Let it dry with a fan blowing on to it for as long as it tells you to. Make sure you are not inhaling the vapors at all and that you can vent the vapors to the outside of the house.

Put on the second coat in. You might be good for years to come.

I have done well with the tape and the liquid; never tried the spray. As above, getting the loose stuff out of the picture seems to be the key.

It will probably work, but the real repair is about as simple as a plumbing job gets. No pressure involved. Sounds like the sink drain gasket is bad. And since you mentioned “grout”, there should not be any there to begin with, so it is an old, kludged repair already. Here is a video showing the details:



As long as the tail stock is not rusted and the trap nuts are not badly corroded it’s a simple, though wet, fix.

That ‘grout’ is probably plumber’s putty. It’s typically applied underneath the metal drain part to provide a seal between the lip of the drain and the sink when it’s tightened to the drain pipe. You may be able to fix the leak by just tightening the drain a little bit more to the pipe. Typically there should be a nut or something on drain pipe below the sink that you can tighten up.

I would strongly recommend fixing this properly since it’s a hidden leak. If spray solution fails, you won’t notice it for some time. A small leak like this would mean the area would stay moist and have lots of opportunity for rot and mold.

I second filmore. This is an easy fix and ideal for beginning DIYers. The only tool you will need is pipe pliers or a large adjustable wrench. Plumber’s putty is cheap. I’m never a fan of simply “covering up” a problem with paint, sealants, or goop.

What CAN go wrong is that the pipe itself might be so corroded that it has no strength and will collapse when you try to remove it. In fact, it’s possible that a pinhole leak in the pipe is actually at the root of the problem. But replacement is easy…just take the old part to your hardware or big box builders’ store.

Tried it on a very old cast iron drain pipe (kitchen sink) in the cellar – slowed it down but still dripped. Plumber replaced cast iron pipe with PVC.

Cast iron pipe was so rusted / corroded you could put your fingers through it by squeezing.

How far above the U-bend (AKA P-trap)? Is it leaking from the union about 6-10" below the sink bowl where the down pipe joins the “U” or from the bottom of the sink bowl itself?

If it’s the former there is likely an obstruction in or beyond the trap letting water back up into the pipe and leak at the union. You would need to fix the cause before fixing the leak. If the latter, you may be able to seal it from below but the issue is likely the drain seal gasket on the bottom side of the sink bowl. In that case the true fix is to replace the whole pop up drain assembly- not a terribly difficult DIY project.

If you do choose to try to plug the leak, I would use a product like Sugru instead of a spray on product. Sugru is shapeable like putty but quickly cures to a waterproof silicone plastic.

I’m gonna Third on Filmore’s post.

In fact, last weekend I just replaced a garbage disposal unit and part of the job includes rolling up a half-inch thick ‘rope’ of plumber’s putty as long as the circumference of the drain, laying it on the sink where the drain-hole is, and then screwing the drain flange in place. The putty squishes down and seals the gap between the sink and the drain flange and will prevent leaks for several years; wipe way the excess* that squishes out beyond the flange.

Once the putty gets old, though, it will start cracking and flaking – and look like old grout. The repair solution: simply replace the putty. It will take you longer to prep and spray the Flex-Seal than to pull pipes and re-putty, and you’ll be doing the job correctly.

That putty is expensive, though. A jar that will serve a household full of sinks (about the size of a single serving of yogurt) will cost you about $5.00 in California. :wink:


  • You WANT an excess to squish out so that, when you wipe it away, you get a perfect edge around the flange. Toss the excess back in the jar and save it for the next need.

I’ve used flex seal tape on a coule of leaks. Don’t try to use the least amount you can get away with to try to save tape. That’s what I did the first time and it didn’t hold. I had to go back and do it again, using a longer piece of tape, so I ended up wasting tape trying to save it.

I’m pretty sure the “right” solution for this problem is to replace the grout. But the problem is that it’s very hard to get into the little under-sink area to actually tighten and loosen things, yada yada, which is why I was hoping for a spray-on solution. After a trip to Home Depot I got some epoxy putty, and I puttied all around the drain. That has reduced the link but not fixed it, next step is to putty around the seal underneath the sink.

(I don’t feel too bad about half-assing it, because we are kinda looking for an excuse to replace the entire vanity and sink, which is 1970s-era cheap crap, so if we break anything beyond recognition, oh well.)