Finishing MDF

On yesterday’s blog, Larry asked for a better photo of the inserted spline, here it is Larry. You’ll notice further down in this blog when the hole is drilled for the hinge insert that you can see the spline there as well.


It’s not as easy as it looks, the smooth flat faces paint fine and do not fuzz or absorb like a machined edge. Here in lies the issue, how to produce a smooth surface on a cut or machined edge. While it’s not difficult, it does require some understanding. First, it is very absorbent and porous thus it will soak up about any thing.

One of the best sealers is shellac, but even then it will absorb a lot of it. Here is a tip….first, sand the area with some 600 grit paper, like sanding an end grain, it helps to seal down the MDF. Then apply your sealer coat, shellac does okay, but In this case I used the General Finishes Under Coater. The under coater is a thick, high solids product made for sealing down MDF, we also used it on the plywood. A good and readily available alternative is Zinssers BIN sealer, it’s a shellac based, high solids primer and does very well. When you apply the first coat of any product don’t expect it to be very pretty, it’s going to raise the fiber and look rough. It’s normal, the key is to let it dry well and then sand it back smooth using  some 220 and then re-coat. The sealer has to harden the fiber and then when sanded it’s hard enough that the sandpaper will shear it off leaving a smooth surface. When you re-coat you may get a little of the same, but usually not much and when sanded smooth it remains. Another big key is to not break the seal coat, in other words, don’t sand through the sealer. Apply some more, you do not want to see the MDF, it needs to be fully sealed in.

To spray the under coater, you will need at least a 2.0 nozzle and a 2.5 is even better. This stuff is super thick. The BIN is also is thick but not as much as the under coater. A 2.0 will spray it well. You can also brush both. I will often brush the edges for my first coat, then after sanding, spray it. The under coater can be thinned with water, the BIN uses alcohol.

Here is a misconception, “Water Based sealers and paints will ruin the MDF.” Not so….water will, but the amount of water in a finish isn’t going to hurt it. Actually, water based products, when dry are more resistant to water than solvent based, again, the key to any sealer is to let it dry well and be sure to get a full coverage coat.



Yesterday, I finished making the doors among other things. I really like the looks of the spline thing and I sprayed a couple of the doors just to show you what to expect with the MDF. Today we need to get the cut areas sanded with some 600 grit and everything sealed. I also have the backs, shelves and drawers to seal, then scuff sand with some 320 grit and they are ready for paint. I know all of this seems like a lot of work and it is….but when complete, we will have a set of cabinets that are smooth, very durable….and will look super! Best get to the gettin’, later ya’ll, be safe.

This entry was posted in Finishing, Furniture Builds, Lumber Types, MDF, Paint, Painted Cabinets, Woodworking and tagged , , , . Bookmark the permalink.

7 Responses to Finishing MDF

  1. Denis Rezendes says:

    nice. i cant wait to see these painted.

  2. Larry Nagel says:

    Thanks Charles for the expanded explanation and pictures. Now I understand why I wasn’t getting it. I now get it!!

  3. Terry Hennessy says:

    Recently Charles gave his impression (review) of the new Woodriver hand planes at Woodcraft. A friend is interested in them but wonders just how good can they be at those prices and I wanted to show him what was said but cannot find that anywhere.

    So, would you please point me to where that was.

    Thanks,

    Terry

  4. Ken Weinert says:

    There’s a little bit on March 12th. I searched the forum and didn’t see anything over there.

  5. Pam says:

    Good information. I’m working on my first piece of furniture and had no clue as to how to finish it. Thanks!

  6. Nice post. I learn something totally new and challenging on blogs I stumbleupon on a daily
    basis. It’s always interesting to read content from other authors and use something from their web sites.

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