Repairing water damage in MDF

Old 05-19-07, 01:25 PM
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Repairing water damage in MDF

Last year I built a large bookcase (14ft x 9tf) with MDF, which was painted with Duron acrylic latex enamel (not the best choice, I now know, as 100% acrylic would have been better). Last month, we had a little flooding in the room above it, and water fell onto the bookcase. Now small sections of the bookcase have some water damage. In particular, in the 14 vertical columns (each of them 9ft tall, about 2 inch wide, made of MDF) I drilled small holes (every inch or so) for the adjustable shelves. Those tiny drill holes didn’t receive paint, and that’s where most of the water damage happened, particularly in 4 of the vertical columns. Now small areas around some of the holes have “swollen” because uncoated MDF acts like a sponge with water.

My plan to correct the water damage is:
1) Sand the swollen area flush. (The exposed MDF will be very porous). The resulting exposed areas will be relatively small, but numerous.
2) Spot-prime with some Duron water-based primer (the same I originally used last year and still keep) in the exposed areas, but this will likely raise the MDF “grain”, which I will then plan to sand. Is an oil-based primer significantly better in this context?
3) Use the original paint (which I still keep) I applied in the small sections I spot-prime, apply one or two coats.
4) Maybe coat the vertical columns with water-based poly for added protection and to minimize blocking. (I did that with the horizontal 1ftx1ft shelves to minimize blocking, with decent results; I understand that the vertical parts don’t have the same blocking problem, but I’d like to do this just for consistency in the texture of the surface).
5) In any remaining exposed uncoated (and porous) MDF section, in both the vertical columns and shelves, put two coats of water based poly for protection and in case of any future flooding. (I know paint would be better, but I want to avoid to make anything thicker: the shelves barely slide into the vertical columns now, and adding more coats will worsen that problem).

Any comments to my plan will be welcome. Thanks
Old 05-19-07, 03:25 PM
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You can sand the swollen areas smooth but they may not be as strong as before. A solvent based primer will do a better job of sealing the MDF but any primer will raise the grain some.

Enamels don't always touch up well and throwing in the use of poly complicates things. You may need to completely coat the boards that are affected. Paint or poly has a very thin film and usually doesn't create a fit problem but if it does, take a belt sander to the shelf edge before sealing it.

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