MDF Sealant Horror

[jclin4]

Re: MDF Sealant Horror

The wood glue probably would stop the paint from penetrating through to the MDF, but I would be concerned about how hard the glue dries. It may tend to be brittle. I suppose there is a fine line on thinning it so it penetrates and not putting it on to thick to avoid to much build up to make it brittle.
Dave

I would not have any concern with the Elmer's wood glue being brittle. I put two coats on the cabs I'm working on last week, not in preparation for painting, but for veneering. Although I'm not painting, the undiluted glue settled into a smooth surface and was not brittle. I used a cheap foam roller and it seemed to seal quite well.

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BTW, the Elmer's white glue worked quite well for veneering (iron-on method). It has a lower re-activation temperature than Yellow Titebond I or II, and there is no expiration timeframe for re-activation. Furthermore, it can be re-activated a number of times.

[lunchmoney]

Re: MDF Sealant Horror

I've used a 50/50 wood glue/water mixture on 4 mdf enclosures now (that I then painted), and it worked extremely well.

It sealed the mdf thoroughly, sanded to a glass-smooth finish, and seemed to be an excellent surface prep for primer.

On my center channel, I tried shellac. Didn't work as well. Sanded terrible compared to the wood glue... balled up and came off in places.

[lunchmoney]

Re: MDF Sealant Horror

The wood glue probably would stop the paint from penetrating through to the MDF, but I would be concerned about how hard the glue dries. It may tend to be brittle.
Dave

Not an issue. I used 5 coats of wood glue/water on my SR71's, and never had a problem.

[brianpowers27]

Re: MDF Sealant Horror

Someone once pointed out that anything water based could cause the mdf to swell. This is the reason that I use the glue undilluted.

[lunchmoney]

Re: MDF Sealant Horror

Please excuse the 'newbieness' of the following questions:

So, what are the final results of sealing mdf vs not sealing? Does it make for a smoother surface? I assume it allows the paint to be applied and dried more evenly. But does it affect the feel or geography of the surface? I can see the benefit of sealing the edges, just wondering of the rest.

I suppose I will need to try this the next time I get a project. It must be worth the effort since it seems popular to do.

Yes, you must seal the mdf prior to painting. Edges and surfaces.

Here's a couple pics of my Tritrix's... wood glue/water to seal (2-3 coats), then sand, then primer, then more sanding, then paint... these are now almost a year old, and still look excellent...

[waynew]

Re: MDF Sealant Horror

If I had a Nickle for everytime I had this problem!

I have used the glue method, sanding sealer, West Systems Epoxy, bondo, shellac, and all sorts of primer...and quite frankly, I am not sure which worked the best. I would say that the easiest to use is the glue on the end grain and avoid sanding the face of the MDF too much. Primer it up big time, and let it dry for several days. The drying time is what I have found to be the biggest component, especially if you live in a humid area. Aaron Hero had a sweet way of dealing with this via a DIY forced air dryer that he used for 24 hours on his speakers...but he is a seriously bad a$$ painter! By that I mean really, really good!

Seems always seem to show up eventually, unless you use the automotive style paint that has a little more give to it than furniture stuff.

[rwrankin]

Re: MDF Sealant Horror

Hey Dave,

I was not intimating that I was blaming anyone for my MDF sealing issue. Sorry if I came off that way! I have always had very good luck with Min Wax products over the years and hoped that the size of my project compared to your totally awesome looking Nightmares might have worked out better. Your Nightmare pictures depicted the desired sealing as far as I could see.
Also, I was trying to sneak reading your post about the build process while at work and may have missed the part where you explained your experience and dislike(s) of the Wood Hardener. I was popping back and forth between several work related windows unfortunately. Sorry, not casting any dispersions your way what so ever. Would never do that, ever.
But after sealing both cabinets using one bottle of the Wood Hardener on each, costly as it was, the sealing produced a pretty nice base for the primer (Glidden's "Gripper") which went on very evenly and should make final layer(s) of paint go on very well.
Time will tell though, because my wife just came up with another of those "Honey Do" projects for me! Looks like I am going to learn (on the fly) how to install a ceramic tile back splash in our recently remodeled kitchen!
Does it ever end with the "Honey Do" stuff?????

[Pete Schumacher]

Re: MDF Sealant Horror

Please excuse the 'newbieness' of the following questions:

So, what are the final results of sealing mdf vs not sealing? Does it make for a smoother surface? I assume it allows the paint to be applied and dried more evenly. But does it affect the feel or geography of the surface? I can see the benefit of sealing the edges, just wondering of the rest.

I suppose I will need to try this the next time I get a project. It must be worth the effort since it seems popular to do.

unsealed MDF can just drink up the paint. Also, unless you seal it up with something, the ends will never smooth out the way you need them if you're after that flawless, smooth, uniform surface. The paper surface of the MDF doesn't benefit nearly as much. However, using the sealant over the whole surface allows you something to sand as you strive for that perfect overall surface.

[Guest]

Re: MDF Sealant Horror

Anyone ever try PVA primer? PVA is what white glue is. It's latex so I don't know if it would make the MDF swell.

[tech9_79]

Re: MDF Sealant Horror

I've read of people using solvent based poly urethane. anyone have any opinions on that? I'm sure it would work just fine and IME it smooths itself out not too badly. I'd be more worried about what ever you're going to put on after actually sticking, be it primer or PVA glue for veneer.

I think you might have something there! MJL21193 over at diyaudio has posted alot on this subject, the baffles he did with the urethane primer, tremclad then poly clear looked great.

[bobbarkto]

Re: MDF Sealant Horror

It's the best thing going if a defect free paint finish is desired on mdf.
Nothing else practical for the diy'er will come close to the moisture excluding properties of a good epoxy primer followed by a good paint.

There are two other products that will do better, 2pk urethanes and aluminized paint.

The 2pk are quite toxic, even deadly. They also don't seal and fill as well per coat and are quite a bit more expensive.
The aluminized paint can't really be topcoated with anything so you're stuck with cheap looking faux aluminum cabinets!

...
My next set I build if I paint them I am thinking about sealing the MDF with an epoxy sealer.

http://www.star-distributing.com/smith/cpeswhy.html

...
Dave

[bobbarkto]

Re: MDF Sealant Horror

Too much and too thick in too short a time.
Thin the first two coats, about 3-1 alcohol to shellac. Let dry a few hours between coats and subsequent coats. Then apply a coat or two full strength.
Before painting or primer let dry overnight - 12 hours or more depending on weather and temperature and ventilation.

You can fix the defect by rubbing with a very dilute coat of the stuff, maybe 10 parts alcohol to 1 part shellac. That should re-amalgamate the crazed surface and remove excess shellac leaving a good surface for primer.

On the recommendation I found, I bought a can of Zinser Shellac Universal Wood Sealant for use on MDF prior to painting.

Why did this happen?


Did I not stir enought? How much stirring does it need? A couple of hours? A couple of days? This crap may've ruined my project.

[lunchmoney]

Re: MDF Sealant Horror

However, using the sealant over the whole surface allows you something to sand as you strive for that perfect overall surface.

I always find it critical to seal both edges and surfaces, since I use a palm sander with heavy grit to smooth out the seams prior to sealing... this obviously takes off whatever smooth finish the mdf initially had.

[brianpowers27]

Re: MDF Sealant Horror

Anyone ever try PVA primer? PVA is what white glue is. It's latex so I don't know if it would make the MDF swell.

I am pretty sure that the stuff I pointed to in post #2 is PVA primer. Results aren't great...

[tech9_79]

Re: MDF Sealant Horror

It's the best thing going if a defect free paint finish is desired on mdf.
Nothing else practical for the diy'er will come close to the moisture excluding properties of a good epoxy primer followed by a good paint.

Is epoxy easy to spray and work with? hows the odor? I hate working with materials that have harsh solvents. And i agree anything 2k just is not worth the brain tumors..:eek:

If the process is simple, doesn't take a month and won't kill me im all about it.