I have hardboard/masonite/composite/engineered wood siding (but it's not LP or Hardie, which I understand are better and more expensive). I noticed a light fixture and the siding aren't very well sealed together. I assume I need to caulk around the fixture? I've never caulked in my life. What kind of caulk do I buy? As to color, could I use a clear one, so I don't have to paint? Thank you]1
The best thing to use is called "Butyl" caulk. It is mineral spirit based (meaning not water based) rubber like caulking so it will adhere to almost anything, it is flexible so it expands and contracts with temperature differences, and once dry, it can be painted. It can also be cleaned up with mineral spirits (paint thinner) if you make a mistake or get it on yourself. Butyl caulking however will last usually no more than about 10 years, so you must check it periodically and know when it is time to re-apply it.
Silicone based caulks will last longer than Butyl but cannot be painted and cannot be cleaned off of anything.
Here is my concern: If you caulk this, it will keep any water or pests from entering, but it wil make it harder in the future to paint or remove/replace the fixture. If you want to caulk anyway, I would recommend an exterior waterproof one. You could also use window/door or kitchen/bath. You might try clear uness you can find one in a matching color to the paint or trim of your house. But, I personally, wouldnt caulk the fixture.