Preserving MDF

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Forum topic by MrRon posted 11-29-2011 08:46 PM 2686 views 0 times favorited 14 replies Add to Favorites Watch
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3898 posts in 2667 days

11-29-2011 08:46 PM

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.

14 replies so far

View Milo's profile


869 posts in 2743 days

#1 posted 11-29-2011 08:50 PM

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

-- Beer, Beer, Thank God for Beer. It's my way of keeping my mind fresh and clear...

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3898 posts in 2667 days

#2 posted 11-29-2011 09:23 PM

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.

View Bill White's profile

Bill White

4408 posts in 3384 days

#3 posted 11-29-2011 10:26 PM

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.


View Don W's profile

Don W

17882 posts in 1992 days

#4 posted 11-29-2011 10:38 PM

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

-- Master hand plane hoarder. -

View TheDane's profile


4939 posts in 3087 days

#5 posted 11-29-2011 10:54 PM

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


-- Gerry -- "I don't plan to ever really grow up ... I'm just going to learn how to act in public!"

View tenontim's profile


2131 posts in 3168 days

#6 posted 11-30-2011 01:22 AM

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.

View usnret's profile


184 posts in 1932 days

#7 posted 11-30-2011 02:32 AM

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.

-- Chief Petty Officer USN(RET) 1991-2011

View SPalm's profile


5249 posts in 3306 days

#8 posted 11-30-2011 02:44 AM

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,

-- -- I'm no rocket surgeon

View doncutlip's profile


2832 posts in 2980 days

#9 posted 11-30-2011 02:52 AM

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?

-- Don, Royersford, PA

View HorizontalMike's profile


7067 posts in 2338 days

#10 posted 11-30-2011 03:42 AM

Another thanks here as well. I am +80% complete on my first MDF mortising jig and was wondering about sealing/finishing as well. 8-)

-- HorizontalMike -- "Woodpeckers understand..."

View MrRon's profile (online now)


3898 posts in 2667 days

#11 posted 11-30-2011 05:37 PM

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.

View DS's profile


2147 posts in 1844 days

#12 posted 11-30-2011 07:50 PM

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.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View Bill White's profile

Bill White

4408 posts in 3384 days

#13 posted 11-30-2011 08:33 PM

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.



View MrRon's profile (online now)


3898 posts in 2667 days

#14 posted 11-30-2011 08:39 PM

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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