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I'm working to replace the exterior pine casing around my front door. I'm wondering if I should paint between where pieces of trim butt against each other, or to use sealant between them, or to butt them together with their unfinished wood touching each other? I'll be using galvanized finishing nails.

My trim needs to be 6.5 inches wide, and is formed with two pieces of wood that run next to each other. Should I put sealant where the two pieces meet, or just put paint at their interface?

I plan to put sealant between the trim and the aluminum siding.

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Your best practice for exterior pine is to prime all 6 sides with an oil based primer rated for exterior. Then join the trim with sealant. Then topcoat with a good exterior trim paint. Take a look at several water bourne alkyd (oil film, water cleanup) topcoats recently available. –  HerrBag Jun 5 '13 at 19:38

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If you're going for maximum lifetime, you should make sure to prime all cuts with an oil based primer. If the lumber didn't come pre-primed, then prime all surfaces with an oil based primer. If you use a fast drying primer, you can get a coat of paint over the cut before you install it. I wouldn't worry about using a "sealant" between the boards. I'm guessing you mean "caulking" when you say sealant.

You should install the trim, and then caulk the seam between the two pieces of lumber after the trim is on the wall. Let it dry over night, and then apply two coats of paint over everything, including any caulking that you've applied. One mistake I see fairly often is that people will caulk something to try and keep water from getting in, but then they don't paint the caulking. The sun dries it up, it shrinks just enough for water to get behind it, and then the caulking has the opposite effect. It actually traps moisture behind itself, slowly rotting the joint that it's meant to protect!

The best thing you can do for yourself in all of this has nothing to do with paint. Make sure you buy solid lumber. Do not ever use finger-jointed pine in an exterior application. I have seen so much rot over the years, because people use finger-jointed lumber in high moisture environments, and it just falls apart in a decade or less, even when it's been painted. That stuff is for interior use only!

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Also, where the casing comes in contact with the ground/concrete/deck, cut it about a 1/4 inch short, and fill the gap with caulk. This will stop moisture from wicking up from the bottom. –  Chris Cudmore Jun 6 '13 at 14:34

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