GRP water tank osmosis

[Toccata]

Is this osmosis?
1997 build Savage Oceanic 46
The GRP fresh water tanks use the hull structure.
The water tanks are under the saloon seats port and starboard, the structure of the tanks include the hull, bulkheads and saloon seat.
I opened the inspection ports, the internal painted surface looked "bubbly".
Saloon seats removed (cut out), painted surface scraped back, flow coat scraped back and the underlying grp is full of white bubbles weeping liquid! Yuk.
I guess the effect can be described as osmosis, or just blistering, each one 3 to 4mm deep. The hull at this point is about 16mm thick.
If ever there is a reason to not build a tank this way, here it is.
Questions are;
Does the grp need to be ground right back and reglassed?
I am fitting free standing stainless steel tanks in the space with plenty of room for air to circulate, can I just leave and monitor, measure the water content with a moisture meter?
The new tank process involves removing a significant portion of the old baffles, (Leaving 50mm against the hull, 2 per side), to fit the ss tanks, so I will be installing a stringer (foam/grp) under the new tanks, fore/aft ie, bulkhead to bulkhead to ensure the hull won't "pant"

[Bill O]

If you didn't rip out the old tanks yet, would consider epoxy coating the entire inside vs. the SS tanks.

Don't know which would be more work, sanding off/prepping the old tanks for food grade epoxy coat.

As you've already suggested w/new stringers, the old integrated tanks are structural so rebuilding that may be experimental to get is as strong as the old tanks.

[BenBowSirocco]

Crack a few big blisters open and measure their depth, if its less than 10% laminate thickness, your probably fine on strength. Your numbers (3-4mm bluster: 16mm hull) are saying 25% of laminate thickness is damaged and at that point, I’m not engineer enough to give you a solid answer.

I saw a similar issue on a glassed in tank I was hired to repair. Ground everything back with a oscillator/carbide tip and glassed over the entire inside with 6oz/epoxy. Heat cured to 120 degree F for one hour.
These were small surface problems and not as deep as yours.

If you choose to epoxy coat:
Use a quality gram scale (in a bag) to make ratios exacting- unused activator is toxic.
DO NOT OVERHEAT Epoxy during post cure, you will damage it- anything above 100 degrees is good, stay under 120.
West Systems has tank building guidelines- although they do not encourage amateur use.
If glassing is not possible: 7 coats minimum is recommended by West Systems. You can used a food safe powder dye to identify coverage between coats (layer to layer) if foam roller coating.

Stainless tanks have their own host of issues especially todays stainless (it seems anyway). Pressure test The tank, and dye test all welds for pockets. Any pockets at all will corrode and require repair at some point. Water will always taste flat and like metal in my experience, but not everyone notices.

Internal tanks are actually really effective and highly efficient things if done properly. But just like hulls they require quality materials and high skill professional building.

[tkeithlu]

That's a lot of fiberglas to be getting anything through from the outside. My bet is that the inside paint is the porous layer, and you've got blisters from the freshwater. No, don't grind into the hull.

It sounds like you have a good plan. Consider scraping the paint from the remaining hull surface, and, as you plan, leave vent space around your new tanks.

In short, my humble opinion is that what problem you had you have solved.

[Cadence]

Originally Posted by Toccata

Is this osmosis?
1997 build Savage Oceanic 46
The GRP fresh water tanks use the hull structure.
The water tanks are under the saloon seats port and starboard, the structure of the tanks include the hull, bulkheads and saloon seat.
I opened the inspection ports, the internal painted surface looked "bubbly".
Saloon seats removed (cut out), painted surface scraped back, flow coat scraped back and the underlying grp is full of white bubbles weeping liquid! Yuk.
I guess the effect can be described as osmosis, or just blistering, each one 3 to 4mm deep. The hull at this point is about 16mm thick.
If ever there is a reason to not build a tank this way, here it is.
Questions are;
Does the grp need to be ground right back and reglassed?
I am fitting free standing stainless steel tanks in the space with plenty of room for air to circulate, can I just leave and monitor, measure the water content with a moisture meter?
The new tank process involves removing a significant portion of the old baffles, (Leaving 50mm against the hull, 2 per side), to fit the ss tanks, so I will be installing a stringer (foam/grp) under the new tanks, fore/aft ie, bulkhead to bulkhead to ensure the hull won't "pant"

My guess is osithalic resin was not used and the bubbles are the MEK leaching out. Once contact with the water is eliminated you should be fine.

[Toccata]

Thankyou for your opinions What an interesting range of thoughts.
Re resin type. I may be able to contact the builder to find out the original build materials.
Scraping paint from various other internal surfaces has revealed a normal looking layer of "resin full" mat with no bubbles/porosity.
The new tanks are underway. A local well skilled manufacturer, even so, I will be on his case to deliver a trouble free product.
I considered a custom polytank, apparently taste is more of an issue than the ss.
I considered a bladder, I don't really like the exit hose originating from underneath (the premade ones anyway) and my shape is awkward, so there would be folds in the bladders to get anything like an equivalent volume (to the current tank)

[Moontide]

Someone once told me freshwater will not blister fibreglass, only salt water.



Anyone have a source for determining the validity or falseness of that info?

[Toccata]

Quote: Stainless tanks have their own host of issues especially todays stainless (it seems anyway). Pressure test The tank, and dye test all welds for pockets. Any pockets at all will corrode and require repair at some point. Water will always taste flat and like metal in my experience, but not everyone notices.

Interestingly, the new ss tanks have just been scrapped due to poor quality ss sheet.
The tanks were on the way to the boat, the fabricator decided to do a last check and removed the protective plastic film. There were areas of defective material, it looked as though foreign material had been rolled into the surface of the ss sheet. Start again!

Re the hull surface. I have borrowed a meter and taken a moisture reading. The hull above and below (in the bilge) the damaged area shows a reading of "0" The area in the tanks shows 20. I think this is a percentage figure. I am going to recheck in a month and see what happens.

[SVHarmonie]

Originally Posted by Moontide

Someone once told me freshwater will not blister fibreglass, only salt water.



Anyone have a source for determining the validity or falseness of that info?

Not only is it wrong, it is exactly backwards. Freshwater causes more osmotic blistering, faster, than salt water. More osmotic pressure to drive the process.

Fresher, hotter water causes more, bigger, osmotic blisters than colder, saltier water.