Sealing Cut MDF Edges

Three different approaches to sealing the cut edges of MDF panels, so finish coatings won't soak in. April 19, 2011

Can someone suggest how to seal the edges of MDF before painting? I will be using water base pigmented lacquer.

Forum Responses
(Finishing Forum)
From contributor R:
Zinsser Bin Primer (with shellac) will work with waterborne lacquer. You may need more than one coat on the edges with a good sanding between coats to smooth the edge.

From the original questioner:
Thank you. Someone told me to try with 50% water/glue.

From contributor A:
1. Sand the edge with 220 grit.

2. Brush 1 soaking wet coat of BIN on the edge. The MDF will soak it up, but the shellac will harden.

3. Sand 320 grit.

From contributor M:
The reason the BIN shellac is better to prime MDF edges is that, unlike lacquer primer, BIN does not dissolve the previous coat into itself, thus causing the whole film to re-soak into the porous edge.

From contributor Y:
The glue/water method is called sizing. I use it all the time on MDF edges, and typically it works very well. I use a water based white glue (Elmer's glue-all) and thin it with warm water. This method does have some drawbacks. It takes a while for the glue to dry, and I find it works best with multiple coats and a fine sanding between coats. You also have to make sure not to get the glue on the non-edge surface because it will make that part extra shiny. Other than that, I think it works great. I use a regular nitrocellulose lacquer though.

From contributor F:
Sanding is the key. The smoother the edge is the easier it is to seal. After routing the edge detail I sand with 220 and then with 320. I use white vinyl primer, put a good coat on the edges, and it will dry in 10 minutes. I then scuff and put a coat on it all. The real key is getting it slick before you prime.

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Comment from contributor A:
We use a paintable melamine edgebanding. We just apply with our edgebander, trim, and itís ready for painting. Itís fast and we donít have to apply any sealer or deal with all of the fuss with PVA, or sanding. Itís really decreased our production time.