# Preserving MDF



## MrRon (Jul 9, 2009)

The project I'm working on, a CNC router uses a torsion box table, because I have to have a flat and stable surface. What can I do to protect the MDF from high humidity? I was thinking shellac or a polyurethane varnish, but I don't really know. Whatever I use, when would be the best time to apply a finish; when the humidity is low or does it matter? I don't want to seal in moisture. I suspect the edges would absorb moisture more than the surface, so would I need to finish the entire thing or just concentrate on the edges? This is a very large project for me, so I can't afford for any warping or twisting which would render the whole project a bust, not easy to fix.


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## Milo (Apr 24, 2009)

What are you going to do with the final product? Paint it? If so, that would probably do the trick.


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## MrRon (Jul 9, 2009)

What should I coat the outside with? I may eventually paint it, but right now, I'm just concerned in preventing the MDF from absorbing moisture.


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## BillWhite (Jul 23, 2007)

I would just shellac the heack out of it. If ya want, apply a couple coat of a waterbased poly. All ya really want to do is seal the mdf. If you really want a bullet-proof surface, use the exterior grade MDF. I used a product (Extira) for some trim. It is tough and heavy. Very stable.


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## donwilwol (May 16, 2011)

i typically use an exterior grade poly, although I agree shellac should work just fine as well.


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## TheDane (May 15, 2008)

MrRon-Shellac. Remember that shellac is a material you can use under virtually any other finish.

-Gerry


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## tenontim (Feb 24, 2008)

I would also say shellac. I have a couple of steam bending forms that are made of mdf and I sealed them with shellac. They are about 6 years old and look like the day I built them.


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## usnret (Jul 14, 2011)

Thin some wood glue with water and apply that to the edged because they will soak up a lot of finish. As for the large faces I use 50/50 poly/mineral spirits just wiped on and let soak in for 10 minutes and remove the excess. Works great for my MDF jigs and never had a problem.


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## SPalm (Oct 9, 2007)

Hey Ron,
Shellac is always a win win. Edges are the biggest offenders, but best to do it all. Don't use any *water based* poly or latex paint.

I have built three CNC machines and learned a lot. You will probably use a spoil board on the bed, and then you level it with the router. It will then be flat and level as far as the router is concerned, so you will be able to recover a slight warping/bowing.

Rick L mentioned a technique above that I really like. Coat both sides of MDF with plastic laminate (Formica). I just used white glue and then iron-on maple edge banding - looks great. This forms a mini torsion box (two skins separated by a membrane). It forms an amazingly strong panel for use in gantry walls and stuff like that. And waterproof. Give it a try.

Good luck and keep us informed,
Steve


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## doncutlip (Aug 30, 2008)

Thanks for posting this. I built a TS outfeed table with an MDF top and never thought about sealing it. I did put edge banding (pine) on it, biscuit joing and glued. But there are still edges on my mitre slots. Time to bust out the shellac again. Does it need to be de-waxed or is it better to leave that in?


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## HorizontalMike (Jun 3, 2010)

Another thanks here as well. I am +80% complete on my first MDF mortising jig and was wondering about sealing/finishing as well.


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## MrRon (Jul 9, 2009)

Thanks all for your time and great advise. Way back before the fancy finishes were invented, shellac was the finish of choice. There wasn't much else available other than paint and whitewash. I guess I got side tracked along the way.


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## DS (Oct 10, 2011)

Maybe consider MDO which is what is used for outdoor traffic signs.
Very sturdy, already water proof and extremely stable. 
Sounds like a good option to me.


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## BillWhite (Jul 23, 2007)

I use an exterior grade MDF called Extira (?). A coat or two of shellac is a winner. This stuff (the Extira) is tough, water proof, and heavy. Look it up.http://www.extira.com/pageBuild.asp?PageID=B_extiravmdf_e

Bill


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## MrRon (Jul 9, 2009)

DS221: The torsion box is already built. MDO costs about 2-1/2x the cost of MDF. I used 3 sheets of MDF. 3/4" MDO costs around $75 a sheet vs $30 for MDF.


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