Fuzzy Finish on MDF

      A smooth surface on finished MDF requires sanding. April 17, 2009

I need some help on how to correct my technique. I recently installed MDF millwork I milled in my shop. My first mistake was I didn't sand it before I installed is, but I had a nice finish so I didn't think sanding was required and the profile is pretty delicate and discrete in some places and I was concerned I'd sand it off. I used a tinted shellac (Zinser BIN) after reading some postings on this site.

I sprayed the material using a compressor driven gravity feed HVLP cup with the pressure set at 50 psi, held about 10" from the millwork. The result was a fuzzy finish on both the milled and factory sanded surfaces. Obviously part of my problem is not sanding prior, but I'm confused by the fuzziness on the factory-smooth surface and thinking I may have made a pressure error, a viscosity error of a spraying distance error. I'd appreciate input.

Forum Responses
(Finishing Forum)
From contributor B:
The problems you mentioned - yes to both. Viscosity and distance are important, and the most important thing is technique involving spraying. Why don't you practice on scrap pieces until you get the look you want? By fuzziness, I assume you mean rough. Speed of hand movement, reduction of material to the type of gun and pressure you're using. All has to do with technique and practice corrects this. I think 10 in is too far, but it depends on pressure and viscosity and material used.

From contributor C:
You have to sand (unless you want a fuzzy finish). MDF is like a sponge and will expand whenever coated with any finish. Furthermore; the individual particles of which it is made will expand at individual rates, creating quite a bumpy surface after the first coat. If these bumps are not leveled you will end up with a very rustic surface.

Furthermore yet again; MDF is normally much denser on the un-milled surfaces so that the milled surfaces become very bumpy and sometimes even have significant pits that have to be filled and sanded to get a refined surface. There are special grades of MDF which have denser cores designed to minimize this but even they will not eliminate it entirely. Solid wood exhibits the same effects but to a lesser extent, so if you want high quality finishes you'll have to learn how to sand, even on complex profiles.

From contributor L:
The thing with spraying BIN is to simply as possible just transfer the material to the piece with as little air as possible and heavy enough without dripping. The stuff dries so fast you are just better off dumping and flashing. I cut it with about 20% by volume with denatured alcohol. Use just enough psi to get it out of the gun, especially for the first couple of coats. Then after sanding you can thin more and use more air to lay out a finer coating. It sounds like you are literally blowing dry some of the material before it even hits the MDF.

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