Repairing water damage in MDF

[Carlos Arteta]

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). 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 against water. (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

[Steve Schoene]

Oil based primer will do little to no grain raising after you have sanded the raised areas flush. You may need to use several coats of primer to deal with the porosity of the MDF.

Then, I think you will get much better results by sanding the entire bookcase and giving the entire surface a coat of a quality 100% acyrlic finish. S-W Pro Classic has gotten good reports in this forum. Even after a year, the old paint won't likely be an exact match anyway unless your paint has very light fast pigment.

It shouldn't be a major task to shorten the shelves enough to make them slide in satisfactorily, so I wouldn't make avoiding that task drive getting a good finish.

[Jim Becker]

You may want/need to skim coat the sanded areas with non-shrinking joint compound/spackle before you prime to restore the smoothness. (A good technique for cut edges on this material, too)