Acrylic latex caulk is not responsive to temperature changes–i. e. , it will not flex or expand as it clings to the bonded materials. It is also not as durable in moist environments. It can be painted over, but in most places it is really not suitable as a caulk for window frames. Siliconized latex caulks have better adhesive properties than acrylics. The best quality siliconized latex caulks are also used as sealants in bathrooms because of the resistance to moisture. Like acrylic latex caulks, siliconized latex products can be painted over. Silicone caulk (different from siliconized latex) is considered one of the most durable caulks available. It is used as a sealant in a wide variety of places, and is especially noted for its water resistance. However, it does not seal as well to wood, and certain varieties even encourage metal corrosion. If you do have difficulties with it, you may be out of luck after you have applied it; it is very difficult to remove. Also, most varieties cannot be painted over. [2] X Research source Polyurethane caulks are considered to be high-quality outdoor sealants, and especially good at sealing together surfaces of different types (such as metal and wood or wood and glass). It is considered relatively easy to work with, but if used outdoors they must be painted over because polyurethanes are not ultra-violet resistant and could break down in the sun. They also are one of the more expensive varieties, and that may be significant if you have a large number of windows to caulk.
