Troubleshooting Thin Build at MDF Edges

Advice on improving coverage of MDF substrates, with a highly technical description of the way surface electrical charges affect the wet behavior of finishes. May 17, 2010

Question
We've just completed a couple of custom pieces where we've done gloss color over MDF. Everything eventually came out well, but we had trouble along the way getting adequate coverage on edges and corners of the carcass and drawer fronts. It seemed the finish almost wanted to crawl away from the edges. It was nearly impossible to sand the faces between coats without going through to the primer or substrate.

For future reference, I'm wondering why the finish seems not to like to adhere to the edges (do we simply need to do a larger roundover)? Are there any tips that will help us build the color more evenly? Our finish is general finishes water-based polyurethane, which we cross-link. Our spray system is a kremlin HVLP.

Forum Responses
(Finishing Forum)
From contributor G:
Sounds like you have to sharp of a corner. Round it over more to just break the edge.


From contributor P:
Sounds like the MDF edge grain is just sucking up the finish, which is the opposite of an adhesion problem. The edges of MDF are very different in texture from the faces, which acquire a kind of glaze in the manufacturing process (which can give you real adhesion problems if you don't sand it).

The edges tend to fuzz up when you machine them, and also when you apply the first few coats of finish.

You could try:
1. Sanding the edges to 220.


2. Applying the sealer or primer extra heavy on the edges
3. Try a different primer like Zinsser BIN.

I've heard of people using glue size (regular PVA glue diluted) on the edges.


From contributor K:
This seems to be an issue more prevalent in water based finishes. I usually means you’re shooting too thick a coat. Try misting a couple of coats on your edges and corners and ten shoot your flats.

From contributor S:
I always dial the fan on my gun down narrow and hit all edges 3-4 times before spraying flats.

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