Sealing the Edges of MDF

Various suggestions for sealing the cut ends of MDF panels. November 13, 2005

Question
What is the best way to seal the edges of MDF before spraying Polystar (white)?

Forum Responses
(Finishing Forum)
From contributor A:
The best I have found has been bondo or spackling paste. It takes a little more time to apply but you are 100% sure that it will never absorb humidity or water. Otherwise if I am in a rush or have too many edges to do, I either spray the edges with shellac or thinned lacquer, 2 coats minimum.

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From contributor B:
You may try yellow glue thinned about 50-75% with water and applied with a brush. You will have to sand between coats. I have used this before on rabbet joints where two pieces of MDF come together leaving one end exposed and it worked very well.

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From contributor C:
If you don't want to band them with veneer, sand them to 600 grit and shoot with 1 coat of MLC Clawlock. Even if you don't want to try the Clawlock and catalyst, try sanding the saw kerf marks out with 150 and then polishing with 280 and then really polishing with 600 to the point where the edge changes color (darker 1/8" layers toward the faces and lighter 1/2" core) and feels glass smooth. If you put any coating on the edge at this point that is not waterborne you will get an amazing edge. The reasons to use any high performance post cat primer undercoater are:
1. Extremely high solids - high build
2. Dries fast and sands easily but gets hard enough that you don't have to be too careful on the edge.

There is no reason to mess around with sizing or multiple seal coats. The only problem with this type of solution (polishing then 1 coat of good primer) is that it only works for a square edge that you can machine sand. Forget getting a really clean look on a moulded cope and stick MDF door. These need to be hand painted after sealing to conceal the MDF edge grain. We finally bought an edgebander that applies a paint grade tape and we use this for all non critical edges. It is very fast, requires no sanding and the glue line hides. Don't make the mistake of building up a 1-1/2" MDF edge with two layers of 3/4" material and then lacquering it. The glue line will witness. Also, don't make the mistake of building up a 1-1/2" MDF edge and then filling it with wood filler, sanding the filler smooth and coating it. The coating can blister, peel and fail. Don't make the mistake of building up a 1-1/2" MDF edge and then edgebanding and coating this - for example, a countertop. It probably won't fail but if it does you have to repair or remove the counter.

The edge tape for MDF that saves all the sanding belongs on the edges of panels or parts that will be lacquered but can be easily repaired and taken back to the site. The best solution for 1-1/2" painted (opaque lacquer) edges is either polished 1-1/2" MDF. (Ultralite also works fine and makes book case installation etc. a little easier on the back) or miter folding 3/4" MDF with a little fill at the miter point.