Unfortunately, I can’t begin to tell you how many times I have gone to a home, given the homeowner a quote to replace their rotting siding, and then a few days later they contact me to say that the cost for a full siding replacement was more than they wanted to spend, especially since a lot of the existing siding “seems to be just fine”. So, they wanted to know what it would cost just to patch it up and leave the stuff that looks “just fine”, figuring it would be cheaper just to replace the bad parts.
This is a very, very bad idea, and there are at least two reasons why. First of all, since there is a strong likelihood that the pressboard siding that “looks just fine” has absorbed water, it is also likely that the load-bearing wood inside the wall (which the siding is nailed to) and your wall insulation have begun to absorb water as well. You know what that means… one of the nastiest words a homeowner can hear… MOLD… not to mention rotting studs. Either one of those issues would drive up the cost of repair, and the longer it is allowed to remain untreated, the more expensive it will be to fix.
Second, because all the pressboard manufacturers were sued and the product is no longer on the market, there is nothing available to exactly match its appearance. So, if you want to replace only the bad parts, you are going to have to find something else that will blend in as much as possible with the rest of the siding that will remain on your wall. There are only a few products that come in similar sizes, and which even slightly resemble the old pressboard siding (such as HardiePlank, etc.) which you can buy in individual planks and panels in different exposures. So, some people get the idea that they can replace just the rotting siding with this other material and no one will know the difference.
Well, guess what? When you see the new material at the lumber yard it may appear that they resemble each other, but as soon as you put them on the wall side-by-side, the difference becomes obvious and anyone can tell that the wall was patched. This happens because the old masonite is retaining water, which causes the surface to swell and its surface texture to change. This means it is less porous and cannot hold paint as well as the new material, so the resulting color and texture difference can be clearly seen, even from a distance.
When homeowners see that there really aren’t very many good options, I often hear one more question: “Then why not just buy some time by painting the old rotting siding?” That’s a BIG mistake… HUGE! And here’s why: when you paint the new areas of siding you’ll quickly notice the difference in how it looks compared to the old siding you did NOT replace, so you’ll probably want to paint the rest of the old pressboard siding with the same paint so it will all match as closely as possible. However, if you paint pressboard siding that has already begun to absorb water (even if it looks “just fine”), the paint seals the siding and traps the moisture between the front and back laminating layers. As a result, the rotting process inside the siding begins to accelerate, especially as the weather gets hotter, so you will find that the time to replace it has come much sooner than you expected, and you will still end up having to replace your old siding, and probably some (or all) of the new stuff you used as patchwork, and the paint job on the old siding will be completely wasted. That’s a lot of money to just flush away for a very short-term gain.