What sealant to use for the holding tank cover?

[SFS]

In my 1983 H31, the holding tank is a molded-in fiberglass tank in the V-berth. It has a very large cover plate that is held on by a LOT of screws and some sort of caulk or sealant. (Sorry, I don't have a picture.)

The sealant is stained in places, and showing signs of past leaks. As unpleasant as the task may be, I'd like to remove this cover, remove the old sealant, clean and reseal the joint, and re-fasten. I'd rather pay this price now, under controlled conditions and with a pumped and well-rinsed tank, than run the risk of an ugly mess sometime down the road.

What sealant should I use at the joint?

Edit: As I said, I don't have a picture, but the cover is not unlike the cover to the water tank, which you can see a picture of in post #1 of this thread:
http://forums.sailboatowners.com/index.php?threads/1984-hunter-31-persistent-leak-near-fuel-tank.181234/&highlight=water tank cover

 

[Crazy Dave Condon]

Is this the only issue. I can help. Let me know what time this morning to call but I am going back to sleep as I have been up half the night with a sick dog.

 

[Crazy Dave Condon]

Is this the only issue. I can help. Let me know what time this morning to call but I am going back to sleep as I have been up half the night with a sick dog.

 

[SFS]

Dave, the PM I sent you was about something else. You can call me anytime at the number I gave you.

 

[Peggie Hall HeadMistress]

What sealant should I use at the joint?

No sealant...make a rubber gasket instead. Plan on replacing it every 5-7 years. You can buy gasket material at any decent hardware store...thickness varies, so you can choose what you need.

 

[SFS]

@Peggie Hall HeadMistress - A one-piece gasket, or cut strips and overlap them? Glue them down?

Seems like a lot less hassle to run a bead of something, but I have better sense than to ignore your advice. I'll talk to you this weekend at the show.

 

[Scott T-Bird]

I wouldn't bother with sealant or fasteners. When I wanted access to the inside of the tank, I cut a 4" diameter hole with a hole saw and used a 4" dia pipe plug, similar to the one shown in the catalog (ABS with rubber gasket). They expand as you twist for tightness and there isn't anything getting by this thing. The plug gasket wedges the plug at the inside edge of the tank, so the cover gasket is pinched exceptionally tight against the tank's ceiling. It is impossible for pressure on the inside of the tank to pop the cover off. It's just as easy to remove. I clean the inside of the tank at the end of each season, after pump-out with a good flushing. I bought mine at Home Depot or Lowes (can't remember which). It's a very simple and reliable solution.
https://www.grainger.com/category/p...e-tubing-and-fittings/plumbing/ecatalog/N-r49
I am referencing the yellow one in grainger's; however, the one I bought is superior to these, I believe.

 

[SFS]

Scott, thanks for the response. I think you may have misunderstood my issue. I already have access to the inside of the tank, though a 6" access portal. I'm not trying to clean the tank. I'm trying to recapture the watertight integrity of the tank, which is (potentially) leaking along 6+ linear feet of edge, where the removable top meets the tank itself. This is, in effect, a repair, not a modification.

 

[Scott T-Bird]

Yes, I now see how your cover is assembled over the entire top and understand the situation. A gasket would be necessary to preserve the ability to remove the cover in the future without having to clean up and re-seal. If you want to make the seal more-or-less permanent, then I would consider butyl tape and make a clean-out access port as I suggested.

 

[thinwater]

I would probably use butyl. It can be laid on thick, is easy to remove, is very lower permutation (some sanitation hoses use butyl liners), and and has the proven chemical resistance (has been used to seal concrete sewer connections for over 50 years, such as manhole covers). I have also used it with holding tanks (about 5 years). Neither polyurethane (and you would never get the cover off) not polysulfide have the required chemical resistance, and silicone is very gas permiable and does not adhere well.

A butly rubber gasket would work as well, though sealing depends on very true, stiff surfaces. Most likely the gasket will need to be soft and thick (at least 3/16", depending on bolt separations). Can't say without seeing it. But I don't see why you would be opening it. Butyl will be easy and sure. Just make sure to use enough so that some squeezes out (not too tight).

 

[fhp3]

I have the same situation on my 28.5 but need to do both the water tank and holding tank. The 30 yr old sealant is peeling up in spots on both tanks. I have held off until I could figure out what sealant to use as well. I never considered Butyl tape for this but it makes sense. On my last boat I sealed everything with Butyl and never had a leak. The stuff is incredible!

 

[Dan Hill]

I had the same issue on our 87 Hunter 28.5 holding tank. Most of the research I found stated that the cover would come out in pieces and would need replacement. I removed all the screws and removed as much caulk as I could from the seam without removing the cover. Cleaned and dried all surfaces using compressed air and "brakleen". Resealed with LifeCaulk, filling the seam and screw holes. Installed all screws while caulk was wet. Never leaked again.

Dan

 

[thinwater]

This is the stuff (same formulation, different size) they use to seal septic tank halves together. The weight of the concrete alone smashes into all the pores and out the sides. Nothing new here, they've been doing it this way for generations.