My boat has a drain plug casing that goes flush into the back of the hull. It screws in with a screw on each side. The drain plug then twist locks into it. When I took my boat out for first time this year this weekend, it had taken on a little bit of water. It looked like part of the seal had dry rotted. I took it out and sanded everything down and reapplied the drain plug case and all with flex seal. I know this isn't the original seal they use but it has worked well with other things so I figured it would work for this too. Just in case I need to reapply a new sealant, what is the best seal you guys have used when sealing up a drain plug? 

Look at 3M Marine 5200. 

I'd use the 3M sealant too.  Btw, silicone sealants can leach chemicals out of plastic fittings over several years and make them brittle.

On 3/27/2017 at 11:41 AM, Bass Turd said:

Look at 3M Marine 5200. 

This is your best option IMO.

The 3M 5200 is great stuff.  I've used a lot of it on my lobsterboat.  But, and here's the rub, you can use it on something that you'll never have to take apart.  There is nothing I know of short of cutting, or grinding, that will touch the stuff.

 

I've use a lot of the various marine sealants in my days as a commercial lobsterman.  For a through hull fitting, I'd use the plain old silicone sealant.  There is a trick to using it however.  Whether you are putting a window in a wheel house, or a through hull fitting, do not tighten them down right away.  Tighten them just enough to get a good spread of silicone between the mating surfaces, then let it sit until it sets up.  Then you can tighten it down and it will not squeeze the silicone out.  You will have literally made a silicone gasket.

 

If you tighten it up right away, you'll squeeze most of the silicone out and it will eventually leak.

 

Whichever way you choose to go, do not fail to seal the tube at the inside of the hull.  

 

One other way is to seal the hole with epoxy.  Get an O-ring that will just slide over the tube, then insert the fitting into the transom and tighten.  That will seal the unit from outside water, while the epoxy will seal the transom from bilge water.

3m 5200