I've used Por15 on a couple of tanks and it seems to work well enough.
Preparation is key so I would recommend the following steps:-
- rinse out with detergent and hot water, usually you will find chunks of rust and other debris coming out. Repeat this step a couple of times.
- next try filling the tank with a couple of handfuls of gravel and shake around until your arms cant stand it any longer. Rinse and repeat a couple of times.
- then get hold of some citric acid crystals which will remove any rust, home brew shops are a source and also eBay
etc. You probably need around 200g for a gallon and this should be enough for a tank. You will need to fill the tank with the citric acid and water, then if you have filled with 1 gallon, position the tank so that the solution covers various areas of the tank for a day or two ensuring every inner surface is covered for at least 24 hours.
- after the citric acid has done its work rinse with hot water and dry out.
- the next step involves using concentrated phosphoric acid (around 75%) rinsed around the tank to form a rust proof phosphate layer. You only need a cup full rinsed around for 10 mins or so, drain any excess. Strangely aquarium shops sell phosphoric acid for lowering the PH of water in fish tanks, so that's one source apart from the web
- next the tank needs drying so you could use a rinse of litre of iso alcohol first and then leave the tank in a warm place with ventilation to clear any fumes.
- lastly use the sealant of your choice, this will involve swilling the sealant around the tank until its completely covered internally and isn't pooling anywhere.
I don't have any particular sealant preference and as mentioned Por15 works well. I have also used Epoxy Novolac Resins similar to Caswell which seem to also work well.