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Old 17-04-2014, 00:05   #1
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Advice on DIY fibreglass holding tank

Well I decided to make a job of replacing the old, leaky holding tank made out of - I kid you not - 1/4" plywood over the coming Easter break. I've made a mold for a 10 gallon tank and fabricated the tank in two halves out of vinylester resin and CSM. The tank halves have a bell and spigot joint that will overlap by about an inch. Despite my amateur efforts, the two halves fit together like a glove. My intention was to use thickened epoxy resin to glue the two halves together, and then seal the inside joint with 2" fibreglass tape.

I think I read somewhere that fresh fibreglass resin will chemically bond to itself for a period of time after it has hardened?? I fabricated the tanks on Sunday night and intend to glue them together tomorrow after cutting the holes for the ports. I'm thinking that perhaps I should use thickened vinylester resin instead. I like this idea because it will both blend in and make the tank look almost seamless and, more importantly, I have vinylester resin left over which will otherwise go to waste. If I go this way, I've got both talc and micro-balloons for thickening. I've heard talc is a no-no for wet applications, but it mixes with the resin much better than the micro-balloons. Even so, is it best to stick with micro-balloons?

Any knowledgeable folks able to shed some light?
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Old 17-04-2014, 15:02   #2
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Re: Advice on DIY fibreglass holding tank

* bump *
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Old 17-04-2014, 15:10   #3
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Re: Advice on DIY fibreglass holding tank

having trouble seeing how you tape the seams on the inside of an enclosed tank?
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Old 17-04-2014, 15:45   #4
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Re: Advice on DIY fibreglass holding tank

Another way to do it is to use the aforementioned 1/4" plywood, seal all surfaces with epoxy resin (you can add white or black pigment to conceal its true origin), then using epoxy with thickeners goop up the corners, drag a beer can over it to create a nice clean radius, sand it and add another coat of resin over the radius. All of this is done before installing the top piece, which has a large cleanout/inspection plate, which then gets a large radius. Drill for your plumbing lines (I use plastic throughhulls) and fabricate the cover for the inspection plate. It should never leak afterwards. My own is older than thirty years and looks brand new.
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Old 17-04-2014, 15:47   #5
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Re: Advice on DIY fibreglass holding tank

There's an inspection port hole. I'll need to do it one handed, but it can be done I think. Plan "B" is to go around the outside instead
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Old 17-04-2014, 16:00   #6
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Re: Advice on DIY fibreglass holding tank

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Originally Posted by Roy M View Post
Another way to do it is to use the aforementioned 1/4" plywood, seal all surfaces with epoxy resin (you can add white or black pigment to conceal its true origin), then using epoxy with thickeners goop up the corners, drag a beer can over it to create a nice clean radius, sand it and add another coat of resin over the radius. All of this is done before installing the top piece, which has a large cleanout/inspection plate, which then gets a large radius. Drill for your plumbing lines (I use plastic throughhulls) and fabricate the cover for the inspection plate. It should never leak afterwards. My own is older than thirty years and looks brand new.
Having just dealt with the old tank, constructed - rather poorly, I'll admit - of plywood I can assure you that psychologically, I just couldn't do it. Also, I recently replaced sections of ply in both my water tanks that were built by the boat builder using a similar technique where it had rotted around the port openings over the years.

Besides, my new tank is already built, I just need to glue it together.
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Old 17-04-2014, 16:00   #7
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Re: Advice on DIY fibreglass holding tank

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Originally Posted by Reefmagnet View Post
Well I decided to make a job of replacing the old, leaky holding tank made out of - I kid you not - 1/4" plywood over the coming Easter break. I've made a mold for a 10 gallon tank and fabricated the tank in two halves out of vinylester resin and CSM. The tank halves have a bell and spigot joint that will overlap by about an inch. Despite my amateur efforts, the two halves fit together like a glove. My intention was to use thickened epoxy resin to glue the two halves together, and then seal the inside joint with 2" fibreglass tape.

I think I read somewhere that fresh fibreglass resin will chemically bond to itself for a period of time after it has hardened?? I fabricated the tanks on Sunday night and intend to glue them together tomorrow after cutting the holes for the ports. I'm thinking that perhaps I should use thickened vinylester resin instead. I like this idea because it will both blend in and make the tank look almost seamless and, more importantly, I have vinylester resin left over which will otherwise go to waste. If I go this way, I've got both talc and micro-balloons for thickening. I've heard talc is a no-no for wet applications, but it mixes with the resin much better than the micro-balloons. Even so, is it best to stick with micro-balloons?

Any knowledgeable folks able to shed some light?
Maybe I'm not that knowledgeable having never used vinylester but have used 100s of litres of epoxy

Stay with the micro-balloons not talc IMO due to the immersion factor.
With epoxy, it will chemically bond up to the stage where you can dent he surface with your thumbnail or put another way, if you can sand the surface without clogging, it wont chemically bond. So if you can sand, you must do so (to get a mechanical bond). Vinylester MAY be the same (but I dunno ).

As to using epoxy with vinylester again I can't help - sorry.
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Old 17-04-2014, 16:08   #8
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Re: Advice on DIY fibreglass holding tank

Yeah, in the absence of a definitive answer, I think I'll go with vinylester. I've redesigned the new install so that the tank is removable for servicing. Worst case scenario I seal the seam from the outside with epoxy and tape at a later date. The other night I had to install a transfer tube between an area that would otherwise develop an air lock (it's a funky design to maximise volume) and used vinylester and micro balloons to install it. Came out looking good and passed the leak test yesterday, so what the heck.
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Old 18-04-2014, 02:23   #9
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Re: Advice on DIY fibreglass holding tank

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Originally Posted by Reefmagnet View Post
Yeah, in the absence of a definitive answer, I think I'll go with vinylester. I've redesigned the new install so that the tank is removable for servicing. Worst case scenario I seal the seam from the outside with epoxy and tape at a later date. The other night I had to install a transfer tube between an area that would otherwise develop an air lock (it's a funky design to maximise volume) and used vinylester and micro balloons to install it. Came out looking good and passed the leak test yesterday, so what the heck.


Definitely stick with vinylester on vinylester. Epoxy often has trouble bonding to vinylester, and VE is more resistant to chemicals than epoxy as well. Rough it up before glassing.
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Old 18-04-2014, 02:27   #10
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Re: Advice on DIY fibreglass holding tank

Thanks Minaret. That's exactly what I ended up doing. Even my wife is impressed with how good it turned out!
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