Quarks Finishing: Sealing the MDF

In which we edge-seal MDF…

Edge Sealing MDF

While MDF is isotropic (i.e. no wood fiber direction), edges are notorious for soaking up a lot of paint. So wherever an edge is exposed, you need to first seal the MDF before you paint it or the paint won’t go on in a uniform way because it just gets soaked up by the MDF on the edges.

The general recommendation is to do this by mixing wood glue with water at a 1:1 ratio and using an old brush to brush the MDF edges. — Since the Quarks are tiny and everything that is worth doing is supposedly worth over-doing, I decided to just paint the entire cabinet.

Not sure I buy the 1:1 mixing ratio for glue to water. The result seemed way too much on the liquid side. Maybe it depends on what glue you are using, but I ended up with something that is probably more a 2:1 ratio of glue to water. With that ratio I got a consistency that reminded me of latex paint (a little bit thicker than milk I’d say…) and that was actually paintable.

Here is what you get right after painting some glue/water mix onto MDF:

Once the glue dries, the painted on layer becomes almost completely transparent. And the MDF gets kind of a nice honey/amber hue. However, I did not get a very smooth surface. After letting the glue dry for 24 hours (a couple hours would probably have been fine) the surface felt quite rough and bumpy. So out came the sanding block and 220 grit sand paper. — Not sure if you can tell but in this picture the cabinet in the front is unsanded and the one in the back is already sanded. (And yes, as you can see, I definitely overdid it with the glue):

I had to put quite a bit of elbow grease into this, to make the edges even again and get a smooth surface. — Maybe I should have done more sanding initially, before putting on the glue.

Anyhow, in an effort to make the seams even again, I sanded right through the glue coating on the cabinet in a few spots. — Therefore I ended up doing the whole procedure of applying glue, waiting for it to dry and sanding it TWICE. After the second time things looked fine. Also, the surface felt really smooth at this point.


As you will see in the next post, the edge sealing seems to have worked. But not sure I would go for glue again. Other people have used shellac to seal MDF edges. — I have some leftover wipe-on polyurethane coating (actually for top coat) and I think on my next project involving MDF I’d probably try using that instead of glue…

The caveat  is that this wipe-one polyurethane coating is water based. So before spray-painting over it you have to make sure it is absolutely dry. To be on the safe side I’d probably give it a couple of days, but that’s something to try for the next time.

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