How Thick?

I remember a flatmate building a tank in GRP. he had to use a pecial hydrocarbon-resistant resin. It ended up about 4mm thick but could probably be much thinner of you use high-modulus stuff, e.g. carbon fibre.
 
You need to make sure the glass is thoroughly wetted out esspecially if you are using chopped Strand mat as it is very easy to get tiny holes which will leak. Roll it and roll it and roill it again. Obviously this becomes a greater problem with thinner f/g. Yes about 4mm might be a good starting point. olewill
 
I would suggest you alternate layers of biaxial glass cloth with medium weight CSM using a fairly standard polyester resin and consolidating well. Don't make it out of all CSM. Try to ensure that you are not overly resin rich, but ensure each layer is fully wetted through by rolling. Dont mix more resin than you need and don't lay up more than one CSM + one Biax at a time. If you do this well a thickness of about 4 mm will be sufficient. I would suggest using a male mould with well radiused corners but dont forget to build in a release angle of about 2 degrees minimum and use plenty of release agent.
I assume you will use an open top with a flange for bolting a flat top lid which will give you access. You can also build all the fillers and feeders into the lid. When you release it, leave it for about 5 days to post cure then clean all the release agent out with acetone and give it 3-4 coats of epoxy on the inside.
 
G'day RECUSANT2,

Have you considered using a closed cell foam or other sandwich, this has the advantage of fast construction and avoids the perennial problem of condensate inside the tank as the sandwich provides insulation.

Tips for tanks:

Build in an inspection port, install baffles, pick up should be 30mm off the bottom, build in a well with a small drain tap, Lay up the inside with at least one layer of roving on top; this will provide a smooth finish with less chance of pin holes and exposed end fibres, fill all corners with micro fibres before glassing, this make for a stronger bob and easy cleaning later.

Hope this helps.

Avagoodweekend......
 
Another way to consider to build a tank is to lay up 6 flat areas of grp to 4mm thickness, for the sides,bottom and top,laminating them flat on an old kitchen worktop or melamine faced shelving..Trim the pieces whilst still 'green'with scissors or sharp craft knife,and then 'tack'it together using small dabs of 2part body filler which quickly sets,then use 4mm worth of strips of wetted f/glass cloth along all seams to assemble the whole.Once cured(4-5 days realistically)you can pressure test the tank,overnight, to say 5psi with a bike pump,before fitting
 
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