Repairing water damaged MDF cabinets

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While building some David speakers, I decided it was a good time to try to fix some speakers I built 20 years ago that were in the wrong place at the wrong time when an aquarium broke. They are made out of MDF and had a nice black lacquer finish.

The speakers sat in the water long enough that the MDF absorbed quite a bit of water and swelled considerably up the sides ~ 6." The seams swelled at different rates and came apart at the bottom and up the sides and the lacquer was also destroyed.

I did not know if it was possible to reasonably fix this, but I figured it was worth a shot.

Here is a picture showing the bottom of a cabinet.

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Here is one showing a little better the seam coming apart.

As a note, the speakers were dried out after 10 years. If this happens to you and you have water damaged speakers (or any other water damaged furniture) you need to let them dry. This can take quite a while depending on the humidity where the speaker is stored. But think in terms of months and months of waiting rather than weeks. You want your wood dry and stable before starting the repair.

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You will notice I cut out cardboard rounds the size of the removed drivers and screwed them on. This will keep all the dust, junk and overspray.

After blowing out the cracks and crevices, the first thing I decided to try to fix things was to fill the cracks that were created by the spreading panels with epoxy resin.

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The epoxy resin is nice, because after you mix it, you have about 15 minutes where the resin will flow quite well down into cracks and crevices.

Before mixing your resin, tape off the areas where you want to put the resin. Hold the tape edge above the level of the area in question to make a well or dam that will keep the resin where you want it and not let it flow all over the place. This is especially true on the edges. Because the resin flows well for a while, you can only fill one face at a time. I started on the bottom because it was the worse.

When you are applying the resin, try to keep it from dripping everywhere. We take scraps off the edge of 1X wood and sand it down so that it looks just about like a tongue depressor and use that to put the resin where we want it. After you have gone around and filled your holes, dents and dings, go back around and check that the resin is still proud of the surface you are filling to. On deep cracks, you can fill it up and it looks good, then a couple minutes later the resin has drained down into the crack and it needs more resin.

You might want to wear gloves doing this. I thought that this epoxy resin was the worse stuff I ever got on my hands until I got into some PL Premium - That stuff is incredible; I swear it prevents your skin from naturally sloughing off.

The resin solidifies enough to where you can rotate your speaker in about an hour and do another side.

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In the picture above, you can see how I put another layer of tape around the top of my first layer of tape so that I could bend it around the rounded corners of the cabinet. This helps spread out the resin where you need it.

Here is another picture showing the edges taped up. My original lacquer job had some tiny imperfections at the panel joints here and there, bugged me for 20 years and I was going to try to fix this too.

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Let the resin harden overnight. The resin will set up hard enough so that you can sand it with a belt sander and it will not clog up the paper.

Lacquer, on the other hand will clog up sandpaper in a heartbeat.

But I started with the belt sander...

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Notice the big light in the background. Back lighting really helps see what you are doing when sanding and finishing.

Here is what happens to belt sander belts when you sand on lacquer for more than a few seconds.

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When sanding finishes, you can wet sand to prevent the sandpaper from getting clogged. You can get a wet rag (wet with paint thinner) and rub it on the belt sander paper and clean it, but when it gets to the point it is above, the belt is toast. Actually the picture shows the paper after trying the paint thinner trick, and then rubbing the paper off with a wire brush.

So time for plan two - paint scraper!

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The paint scraper you get at the store is not ready for the job we have in mind - it must be sharpened. We use a regular file to put a much steeper angle on the blade than they come from the store. It takes a while to learn to sharpen the scraper, but once mastered it really makes for a wonderful tool.

When sharpening, work the edges more than the center so that there is a very slight arch across the blade. This tool can do unbelievable damage if a corner of the blade digs into your work piece.

We just set the scraper on the edge of the workbench or counter and run the file in a downward motion to sharpen the blade.

We find the 2" wide blade the best - 3" wide versions are not as efficient.

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Here is a good shot showing the damage on the finish.

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Note the top handle knob over the blade - very useful for applying the pressure needed to scrape the finish away.

A sharp scraper will pull heavy shavings in oak. My dad and I would use these scrapers to strip a medium sized kitchen and sand it by lunch time. We had two kinds of customers - those who had tried stripping their kitchen and got one end panel done in a weekend; they were in awe as we came in with $10 paint scrapers and wiped out there kitchen in hours, and those that just called us in to do the re-finishing, saw us whip through the old finish and wonder why we charged so much, for something so easy.

When scraping stuff like oak, we spend as much time getting up and sharpening tools as we do actually scraping. Really, ten-twelve good pulls then more sharpening.

Here you can see the shavings of the hardened resin.

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Scrape all the finish away so that the wood is exposed and the belt sander can be used without clogging the belt.

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The wood is swollen so it needs to be sanded down flat. You can tell how you are doing by looking to see if you are getting into the remaining finish. Blend in the bare wood to the remaining finish.

Be very careful using the belt sander. It can dig deep gouges very quickly if not held very flat. It would be much safer to get it close with the belt sander then take it the last bit with a random orbit sander.

Use a random orbit sander to sand out the proud resin on the edges as well.

After getting things flat, it is time to work the rounded edges. You could hand sand (sllooowwwww), use a pad sander, (slow), random orbit (risky) or use the router bits you used in the first place - fast and safe.

Big round over on the front, only a 3/4" on the back.

Had to dig out a router I did not even know we had to fit that big bit.

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Here are the edges after the router. Went a bit deep on the big bit, not happy with what was happening with it set flush.

Used a random orbit sander to blend in the little ledge left by the big bit.

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Then used a pad sander to smooth out the edges.