Sounds like you've already solved the problem the best way. For anyone who may reference this post in the future though I have to agree that Caswell's is a good product. The most critical things are getting your tank as clean as possible then mixing the proper proportions. Really not a difficult job IF your tank is not FULL of flaky rust. Follow the directions and if you have any issues contact them! Caswell's is used by some pretty esteemed fiberglass tank makers. That was what made me choose them. No regrets!
That said, I believe their smallest kit (more than enough for one tank) is currently about $75-$80 shipped. So figure $100 for labor. It is a several hour job not counting cure time. $175 for clean and seal I'd say isn't so bad a deal. Not to mention IF something does unexpectedly go wrong, it's on the shop instead of you to do it over!