Are you using power tools or hand sanding (which would indeed be tiresome - but you'd have some serious guns afterward ;)) to remove the heavy rust? Eastwood has a tool that's supposedly pretty good at removing rust, but I haven't used one. I have used an angle grinder with a flap disk on it, and it has taken me only about an hour to strip an entire front fender.
Like Kevinski suggested, Evaporust is a good product. Use a towel to keep it wet so it can work. I've tried their gel product but with so-so results.
