Re: How to fix pinholes in base corrugated water tank?
Welcome to the Bunnings Workshop community @Dianne608. It's terrific to have you join us, and many thanks for your question about sealing a water tank.
Could you tell our helpful members a little more about what the tank will be used for? That way, they'd be much better placed to offer useful suggestions.
With multiple coats of sealer, you should be able to cover small pinholes, but the job of most sealers is to provide a waterproof layer on an already solid substrate. They are not necessarily designed to plug pinholes. If this is for potable water, I recommend you consider using Crommelin 1L Exterior Grade Brushable Waterproofer. This product should seal pin holes with a couple of coats and is suitable for painting the entire interior with. If the pin holes are quite large, this product can be combined with the reinforcing fabric.
It will prevent the water from contacting the steel and help prevent further rust from occurring.