Thanks for all the suggestions regarding water retaining materials and chemistry, I have, after a bit of a chat with Alex the Techie, who works for the GRP suppliers, opted for vinylester resin as a top/gel coat.
Apparently its a much better product than polyester based gel coat and will cope with water at 110c plus its a lot more resistant to chemical attack and only £11 a litre including vat, activator, some blue powder and a splash of wax to stop the outer surface being a bit sticky.
So finally, down t'pit for fun n frolics -
Preparation of the surfaces has been pretty miserable and a belt sander with 60 grit was utilised.
I always wore a fresh dust mask, as once the glass strands get exposed theres some pretty small particles on the move and some seriously itchy arms persuaded me to buy a handful of 'all in one' goon suits.
There was one very obvious brown stain had appeared halfway down one side, indicating a pinhole, so I rubbed the area back to glass mat, in readiness for a patch.
The bottom of the tank with all its festering blisters was dealt with by using the heat gun. This quickly identified the areas where the gel coat had separated from the underlying GRP as the trapped liquid steamed out and the gel coat curled and cracked. No amount of belt sanding was going to level up the resulting craters though, so I decided to build another good layer of glass mat and resin, to level it all off enough for the gel coat to stand a chance.
Next challenge was trying to track down some acetone to wash down all the prepared surfaces, as recommended by Alex the Techie.
This took a couple of excursions on my motorbike ( the weather being a bit nice and the pit being less so ) and on inspiration I phoned the Lard himself to see if he had any, or knew where to get some.
He was just setting out on a bicycle ride to a local vineyard and hinted at the route he would be taking .....
Ha, I could intercept him and inspect those wobbly legs ... or would they be covered in Lycra ...... ooh, what colour Lycra does he cover himself in
?
I rode like a madman but sadly our two wheeled tryst was not to be. Perhaps he hid in the ditch as I stormed past, not wanting the good Navvi folk to know what I would be obliged to reveal .....
Anyway it turns out that apart from Boots the Chemist, who will sell you a few cc of nail varnish remover for £2, no one sells acetone anymore as its a controlled substance or summat complicated. I did eventually get lucky at a car paint suppliers, who sell acetone as 'fast aggressive thinners'. It smells right and is identified by numbers only.
Back home and I had to consider how I would survive in a 2m deep pit loaded with acetone vapour.
The 4" in-line fan attached to a 3m length of elephants trunk, that I used before, was currently in service for dust extraction and air circulation for the rubbing down, so I cleaned out the duct, reversed the fan and created a lovely air feed to mouth and nose using a 1 litre lemonade bottle and some draft excluder, held in place by the bike goggles, with strap at front and gogg at rear of head. Awesome ! I even cut some slits to blow fresh air past my eyes as acetone and styrene are pretty harsh. Yeah its a bit cumbersome but it works a lot better than the mark 1 and it simply blows loads of fresh air exactly where its needed.
All was eventually looking good and ready for laying up the glass mat and resin on the bottom.
Using the resin and activator that I had previously used, I boshed it down on half the base area, ( its pretty tight down there at 2ft x 4 ft ) and waited, and waited .... No amount of heat or waiting was going to get the resin more than a bit set though.
from Alex the Techie - Tetrosyl may use BPO hardener rather than MEKP which could explain why its not going off. If that's true it will never go off properly. Such are the conditions this tank has to work in, if you cant get it to cure you will, I'm afraid, have to remove it.
Doh ! bl00dy chemistry again.
So how come it went off last time using the wrong hardener
Hardeners are organic and 'go off' apparently
Odd that in places its gone off ok then ....
It was still a bit mobile today but I managed to do the other half using some fresh, correct hardener and have overlaid the semi set stuff with a good coating of resin that does set.
Hopefully once it starts to cook it will trigger a set right through but I am not all that confident.
If it does, then I am ready for the final gel coat, although I have a cunning plan for maintaining the water level in the tank that I may execute.
