If the job is done hastily or without proper preparation and care, applying caulk on the exterior of your home can be a real mess. All of the old caulk must be removed before applying new caulk (silicone/silicone sealant/retaining sealant/potting sealant/silicone adhesive). The use of an oscillating tool with a flexible scraping blade works well for this task. Caulk is runny, sticky and the material dries at different rates depending on temperature and humidity. Once the caulk is applied, it must be smoothed into the joint or crack, or else it may run out or dry unevenly.
The best tool for smoothing over caulk is also the simplest: your finger. The most important thing to remember when smoothing caulk with your finger is that it is extremely sticky; if you simply stick your finger in there, you are likely to pull out all of the caulk as it sticks to your skin. You must wet your finger first. Dip it in a small cup of water before smoothing. You can also wrap a thin, damp paper towel around the tip of your finger before smoothing. The water will ensure that the caulk sticks to the dry surface to which you applied it and not to your skin.
There are also a number of professional tools you can use to smooth caulk. If you are having trouble with excess caulk around the edges of your application area, a jointing spatula will allow you to scrape the surface around the caulked joint without gouging out the caulk itself. You can also use a jointing tool, which will smooth caulk like your finger. The jointing tool will also make a small trench in the caulk, setting it slightly below the surrounding area, like the mortar lines in a brick wall. You should also wet these tools before using them to make cleaning off caulk easier, or to stop caulk from clinging to them to begin with.
If you don't want to get your finger wet and dirty and don't have a professional tool available, you can turn nearly any flat or convex object into a smoothing tool. Try using Popsicle sticks, butter knife, the edges of soup spoons or cooking spatulas to do the job. Again, be sure to wet the surface first. If you find caulk sticking to any surface you are using, try wetting it down with water mixed with dish detergent for a more slippery surface.
No matter what you use to smooth your exterior caulk (silicone/silicone sealant/retaining sealant/potting sealant/silicone adhesive), follow some basic steps to ensure a secure hold. Always smooth the caulk in the same direction that you are applying the caulk. Smooth towards areas to which you have not yet applied caulk. Smoothing back towards the already caulked area will only cause bumps and ridges that will create an uneven caulking surface.