Hello Customer (name blocked for privacy) and welcome to JustAnswer!
You may use silicon caulking (made for outdoors use) which you may find in every hardware store. This sort of silicone will stick on almost every sort of construction material and will resist the weather conditions. Just to give you an idea what I am talking about, here's a link to such a silicone caulking seller, but you may find another brand in your hardware store:
http://www.rutland.com/care1/care1.htm
Silicone is elastic and will compensate the expansion/contraction of 2 different materials with changing temperatures. They usually come in white, black and clear colors, you may choose whichever you think is best for you but you may also paint over the caulking however the paint will not stick very well.
Caulking is good for this purpose because it will seal the gap and prevent water, dust, moisture, bugs etc.
Hope this helps but if you have questions please let me know.
Have a great day!
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