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Area is US Pacific NW (Portland, OR)

My recently purchased home (built 1972) has clapboard siding made of (I've been told) Masonite, probably 10-15 years old. When it was installed the painters painted under the butt edges only where it was easy to get to, i.e. knee high and above, and did not paint under the bottom two courses. This has resulted in some water damage over the years.

See below for pictures.

I'm not in a position to replace the siding or do major repairs, but would like to prevent or at least slow down further damage by sealing the unprotected butt edges.

I am considering applying a clear paintable silicone caulk to the underside of the butt edges on the lower two courses, and any other butt edges that are incompletely painted. Would this work, or should I be doing something different?

Wide shot of the siding

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Butt edge about 6' up. It is mostly painted but even this one could use some sealing

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Bottom course, this is the worst of it.

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Bottom course with less swelling/damage.

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  • Hardboard doesn't have a long life. It's cheap and fairly easy to replace. If you're going to all that effort, just pop on some new boards. Seal them up well beforehand and you should get another 15 years. – isherwood Jul 18 '16 at 20:26
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In my area, Masonite siding was extremely popular during the 70's and 80's. They all have met the same fate as what you are experiencing. Your's doesn't look too bad yet, but once it starts, the bubbling and swelling will continue. The only way I know to slow the process is to remove as much loose paint and disintegrated paper material as possible, prime with a sealing primer like Bullseye pigmented shellac, or Kilz. The goal is to be sure you have sealed as much of the affected surface area as possible. Use two coats of primer if you want a better seal. Get up behind the slabs as far as possible. Then overcoat with your color paint. Try to do this when the conditions have been dry for a few days and the siding is as dry as possible. If it is damp or saturated with water, like I have seen many times, painting is a waste of time, it will not stay and the swelling will continue. Keep a good coat of paint on any areas of wear or where chipped or peeling. You cannot eliminate the damage without replacement, but you can slow it down.

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