Caulking is one of those professional tricks that can really boost the longevity of your exterior painting project. Caulk is a glue-like substance that seals cracks and seams, like those between the siding and windows, or the siding and trim of your house. The enemy, as usual, is water, which can seep into those seams and cause your brand-new paint job to crack and bubble. Caulking is different from patching, because you will need to caulk even undamaged areas where water might seep through.
Caulk should be applied after the primer stage of an exterior painting project. The experts recommend using a siliconized acrylic caulk because paint will stick to it [source: Hurst-Wajszczuk]. If you are caulking an area between two different types of surfaces — wood and concrete, or concrete and metal — look for caulk that's compatible with both materials. Don't try to caulk cracks between clapboards, though. Moist air from inside the house needs a way to escape [source: Barhnart et al.].
To apply caulk, you need a caulking gun, which is an inexpensive metal applicator. Apply a thick line of caulk over the seam and run back over it with your forefinger to create a smooth, flat seal. If the seam is really wide, you might need to buy some backer rod, a foam-like tube that you insert into the seam before applying the caulk [source: Truini].