Corroded holding tank access plate

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catalacmarc
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Corroded holding tank access plate

by catalacmarc » Fri Sep 13, 2019 12:56 am

The eight inch square aluminum plate through which the plumbing for the holding tank runs has a major problem. A brass or bronze elbow was installed for the inlet. I discovered yesterday, that around the yellow metal elbow, the aluminum plate has corroded right through. (see picture) I have made an epoxy and fiberglass plate to replace it. My question is what are the bolts that hold the plate down bolted into? Thanks Marc
20190911_114304_resized.jpg

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chicagocat
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Re: Corroded holding tank access plate

by chicagocat » Fri Sep 13, 2019 10:40 am

Hey Marc,
This obviously looks like classic galvanic corrosion. Neither of my 36s has a metal holding tank. They're both some type of plastic. What hull number is yours, BTW?
The bolt question is difficult. They either have washers and nuts on the back and maybe the plate was welded into place later in some way I can't see in the picture. Or the tank itself could be threaded with a tap. That seems unlikely, due to the thin nature of aluminum tanks.

It seems to me that replacing the entire plate may be in order, which would be an ugly job, but do-able.

Since you've already made a fiberglass plate, West Systems says that their G-Flex product will adhere to aluminum.

I have to think on it and see what others say.

Brendan
PDQ 36052 - 1996 LRC - "Anne Z" - Chicago
and 2001 PDQ 36 Classic (Tall Rig)- "Cat Tales" - Punta Gorda, FL

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thinwater
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Re: Corroded holding tank access plate

by thinwater » Sun Sep 15, 2019 9:42 pm

The tank appears to be fiberglass; you can see the fabric.

For the record, aluminum is a terrible material for holding tanks and anything related to sewage. Urine eats it up. The brass didn't help.
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catalacmarc
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Re: Corroded holding tank access plate

by catalacmarc » Mon Sep 16, 2019 8:39 pm

Thinwater, amen to that. The tank is glass with the aluminum plate bolted through. Somebody years ago had plumbed it with that bronze/brass elbow. I had hoped to be able to remove the plate but the bedding will not release nor soften with heat. Current plan is to sand around the plate, cut it out and use a 10.25 inch fiberglass plate epoxied to the tank to cover the opening. Will use PVC pipe and Sealand rubber bushings for tank inlet outlet and vents. The Sea land products are nifty, they will allow me to a cut holes in the fiberglass plate, push in the bushing and then push PVC pipe through the bushings expanding them to seal the tank.
Marc

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thinwater
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Re: Corroded holding tank access plate

by thinwater » Tue Sep 17, 2019 6:17 pm

They probably used 3M 5200 or a similar polyurethane. Try Re-Move / DSR5. Apply, slit along, the seam, recoat, and wait. I would try this before cutting.

Uniseal (https://www.usplastic.com/catalog/item. ... emid=89951 is the one I am familiar with. But I would try Re-Move first. If you replace the plate, use silicone.
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Re: Corroded holding tank access plate

by catalacmarc » Tue Jan 21, 2020 2:09 pm

This is an old topic but I never finished it. The Remove suggestion was miraculous. I used the locally available version to remove old excess caulking around the plate prior to trying to slice it it off the tank then neutralized it. Came back the next day and the whole old aluminum plate lifted off the fiberglass tank! No slicing of the old caulking required. West Systemed a new fiberglass plate on the tank with push through fittings.

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Re: Corroded holding tank access plate

by thinwater » Fri Jan 31, 2020 10:55 pm

I'm glad the Re-Move worked out. I enjoy testing things so that we can all benefit. Many products say they will remove polyurethane cauk=lk, but only Re-Move really does.
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