I've used many methods for not enough hardener, especially when cooler weather hits. But the hairdryer method works really well, and also applying a few drops of the hardener (2 or 3) directly to the area that feels tacky and rubbing them all around the affected area with a gloved hand also works really well, and doubling up on the hardener when it gets cold outside also helps to get a good curing.
Creating a microclimate works well too which you can do by either building a wooden box and cutting holes for spot lamps with heat bulbs (pet store) and leaving your part in the box for about 10-20 minutes, or by setting your oven to 120°F and throwing your part in for about 10 minutes (use this method at your own risk as the health risks of cooking in an oven that you use to cure resin in are unknown, and it makes the kitchen and most of the house smell like resin; just because I've done it doesn't mean that you should!).
Another way if you have an electric stove or a portable electric element is to turn it on to high heat and hold the sticky part 4-6 inches over the red hot element, it will very quickly raise the temperature of the uncured area to curing temperatures. Be careful with this method as you need the part to reach 120°F (hot to the touch) but any higher and you risk fire, burns, or part warping. The part may discolor when using this method, it doesn't seem to affect the overall strength of the part tho so try not to freak out if the resin turns brown, just get it away from the element and let it cool. When it cools it should be cured.
The thing behind polyester resin, bondo, and even a lot of smoothcast resins and some silicones is that they are thermosetting. That means that they need some type of heat to polymerize and become rigid. The hardener you add (methyl ethyl ketone peroxide [MEKP] in the case of bondo and polyester resin) creates an exothermic (heat producing) reaction when combined with the monomeric resin in the right proportions, which is why resin and bondo get hot while they cure. Adding too little to the resin or leaving the part in a cool or cold place to cure results in not enough heat to polymerize the resin quickly so the trick is to somehow add heat to aid the hardener in the creation of the polymer chains.