My vertical composite siding is starting to pillow outwards on the bottoms. See LP siding.

How can I seal it for a few years until I replace it with vinyl siding?

Here is a photo from the web.


  • A little known fact in the future of most of the McMansions that have everyone so steeply under water is that they were built with this stuff. If you don't keep a good, thick layer of semi-impervious paint on this stuff, you'll soon have a $450k granite countertopped, faux-stone clad, tile roofed wonder with exterior walls making all those T-111 sided cheap ranch styles looking really good. Also, immediately take care of any gutter problems or rain splash issues as it hangs at the bottom of the boards and works its way up through the material. Commented Jul 26, 2014 at 14:31
  • Give it 25 years from install till replacement, we're getting ready to rip all of it off our company's building. You can push your hand through it now. It does not stand up well to Oregon's moisture, even if you keep a good coat of paint on it. While the resins used to bond the wood fiber together can be marine grade waterproof, the moisture soaks into the material through exposed wood fiber at the bottom and capillaries up into it. The OSB core rots from the inside-out. Commented Mar 23, 2015 at 15:07

2 Answers 2


First I would determine how old the siding is. If made in the last 20 years it's probably covered by manufacturer warranty and replacing it with vinyl would be a step backwards.

Also lap siding isn't supposed to seal on the bottom. It's supposed to allow any moisture or condensation that happens to get under the boards to drain downwards. So you should not try to seal the bottom edge of the boards, at least not with anything that is impermeable to moisture.

If the house's sheathing isn't wrapped in something like Tyvek then significant gaps at the bottom of lap boards could give an opening for pests. If you're having pest problems you might just address those.


There really is no repair once hardboard starts to decay. You could paint it to prevent some future moisture intrusion, but it won't make the siding more solid.

A good quality latex paint will add a bit of life to it. Whether that's worth the cost and effort is debatable. I'd replace any seriously rotten boards and ignore the rest until you re-side.

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