PJ,
I am not familiar with Petseal, however. I suspect it will setup in time. Two-part systems often take longer than advertised to harden, particularly if the ambient air temperature is close to the lowest recommended temperature allowed for use (often, around 50 deg. F.). If you are within the allowed temperature, be patient, I would not accelerate with a heat gun. Raising the air temperature in the room would be better. Assuming it hardens and still leaks, I would go with Caswell's tank sealer shown at
http://www.caswellplating.com/aids/epoxygas.htm. I wish I could have recommended this to you before the fact. I used it and am very happy. However, I only have the one experience with the product and it hasn't been in place a long time. I believe there are some more testimonials on the Caswells site. For those of you who may not be familiar with this company, you will find their do-it-yourself chrome plating kits very interesting.
If the product does not harden, we have a problem. At that point you will need to determine if there is a solvent that will dissolve the semi-hardened muck, then flood the tank with the solvent, allowing soaking time. All this is said with strong cautions. Whatever solvent, if any, that works will be very volitile and noxious. Please be careful. Test the usefulness and result of the solvent on a small area. Be sure you have plenty of ventilation and, maybe, are not breaking the law for chemical usage in your area.
Richard