Priming MDF Raised-Panel Doors

Primer can do unpleasant things to the exposed wood fiber of routed MDF. But there are quite a few ways to prime machined MDF beautifully. August 30, 2005

Does anyone have any tips for getting the primer right on one-piece MDF raised panel doors? The areas where the CNC router bit carved out the panel and the ogee edge are obviously not as smooth as the face of the MDF.

I've tried what an ML Campbell rep advised me, which was to sand up to 400 grit in those areas before the first coat of primer. It still fuzzes up a lot, even on the second coat. It just doesn't seem right to be laying up to four coats of primer to get a smooth substrate for the lacquer - that's a lot of sanding. Any help is appreciated.

Forum Responses
(Finishing Forum)
From contributor D:
What kind of primer are you using? WB or solvent?

From contributor G:
After you sand the profiles, are you blowing the dust out with hi-pressure air? That just opens the fibers up again. Try a brush.

From contributor J:
You may want to try Zinssers De-waxed shellac. Other suggestions I heard of have been to apply a light coat of drywall compound or spackling around the edges, let it dry and sand off. Make sure you are using ML Campbells clawlock.

From contributor W:
Have you ever tried using sizing before priming? Give it a try. You can get it from Franklin (Titebond). Don't make the mistake of just watering down glue - get the real thing.

From contributor R:
To contributor W: Could you explain your reasoning behind not using a regular watered down glue for a sizing. I donít use any water based products but the regular white or yellow glue has worked great for me on solvent based coatings for nearly thirty years.

From contributor W:
The real sizing has additives in it which prevent a crackle from springing up. While it may not affect those of us who use industrial types of finishes, if a regular paint was used directly over the glue, it makes for a crackle finish.

From contributor M:
To contributor W: I have been using white glue with water for years to make up a size. I have used this barrier coat over and under almost every coating. I allow it to dry, and never once did it fail me. I have used it to prevent blotching, and as a size on MDF I have used it to seal in silicone. I have even used all the glues to create crackle finishes, etc. I don't think it has anything to do with us using industrial finishes. White glue works as long as you let it dry - air or forced dried.

From contributor R:
To contributor W: I can understand finishers who just go by what a manufacturer tells them, but Iím really looking for hands-on experience from finishers who have used regular glue and who have had problems with it.

From contributor S:
I have used yellow and white glue thinned with water for years with no problems. The important part is thin if you would apply straight glue you will have problems. I also do crackle finishes using hide glue mostly and the difference is the glue is applied over a full coat of paint or finish not on a raw substrate.

From contributor W:
The first time I tried to thin the glue, I guess I did not add enough water. It made a big mess of my project. That was when I called the tech dept at the glue company. They suggested that I try the sizing because it had an additive to prevent crackle or crazing. I called my local lumber supplier (who supplies my glue) and asked that he stock the sizing for me. For the minimal cost, I just buy the sizing.

From contributor T:
I would recommend Becker Acroma's Bernyl Surfacer/Matador. It is a post catalyzed lacquer system that will offer you excellent filling properties. A pretty standard system would be two coats of primer with one top coat.

The first coat of primer should be sanded very aggressively to the point of the board beginning to show through in high spots (240-280 grit). The second coat should be sanded to achieve the desired degree of smoothness (280-320 grit). Finer grits or another top coat may be required for higher gloss finishes.

From contributor H:
Heavier, denser MDF panels finish better. Sanding is another very important key. Lastly, higher solids in the primer coats fill and cover better. 45 lb MDF, sanded at best, and used with a high solids primer will bring you to a higher quality finish faster.

From contributor A:
We struggled for years with different products, grits, and etc. We primarily use MDF for raised panels in doors and wainscoting. Our current protocol is to sand with 220 grit, edge-prime with BIN (white shellac primer), then sand 320, and prime with a thick coat of ML Cambell Polystar WB surfacer. The latter is just like thinned drywall compound.

From contributor M:
To the original questioner: If itís on closed grain woods, I use 1 part glue to 3 parts of water. On open grain woods I use 50% glue to 50% water. I brush on the size and allow it to thoroughly dry. I also use a hair drier if I'm in a rush.

I always give it a second coat to make sure I covered the entire part or piece. You don't want any holidays, as the coating can get in between the sizing and the substrate and cause other problems. Be sure it is thoroughly dry before you move on.

From the original questioner:
Do you apply it to the entire surface of the MDF, or to only the surface of the cut edges (the profile)?

From contributor O:
I would not waste time with glue sizing when you have advanced primer undercoaters designed to do the work for you. I am switching away from solvent borne coatings but can tell you that as far as the MLC Clawlock product you mentioned goes we have been sanding the MDF to 600 for years and then a single coat of Clawlock generally looks like Corian.

If you want to know how it works do this a test along these lines with MDF:

1. Polish a rough, saw kerfed piece of 3/4" MDF (right out of the saw) with 600 until the cross section of the core (inner 3/8") turns a different color from the outer bands nearer the surface. It should be as smooth as glass but you won't be able to take out the blade kerfs.
2. Do the same thing but take the kerf marks out with 150 and then go to 600.
3. Use the 150 first and then finish with 320
4. Use the 150 first then finish with 400

You should have four samples to edge prime:

1. Saw then 600 full polish
2. Saw then 150 (eliminate kerfs) then 600 full polish
3. Saw then 150 (eliminate kerfs) then 320 polish
4. Saw then 150 (eliminate kerfs) then 400 polish.

Try to shoot the edges with the same amount of Clawlock. Your results will probably be the following:

Both 600 polish will be great, but the one that came right from the saw shows the kerfs. The 400 will be almost as smooth and the 320 will probably cry out for another coat.

What will this show you? It will simply show that if you want a one coat primer solution for smooth MDF it is available with Clawlock, but you have to polish the fiber. Itís not that hard on the square edges, but it may well be on a moulded profile which you might risk destroying. Your options could include glue size, but even if it could give you high solids it won't sand like a primer, dry like a primer or ensure systematic adhesion like a primer. Another option would be to consider a different material to mill.

From contributor J:
To the original questioner: On MDF I sand the edges with 120 then 150. Then I spray on a premium Alkyd primer (quick dry). After it dries I sand lightly with 220, blow off the dust and then wipe with a tack cloth. Then I spray two coats of premium oil base enamel, sanding (very lightly) with 220, blowing off the dust and wiping with a tack cloth between coats. It usually comes out looking like glass. Also make sure to spay it very well.

From contributor B:
What is it that you want to spray - the primer or oil base enamel?

From contributor J:
I spray the primer. I spray it (the primer) on as heavily as I can without making it run, but even if I get one or two they're easily sanded out of alkyd primer. I go easier when spraying the oil base paint, thinning if necessary. If I want it extra nice I'll spray a third coat of paint.

From contributor G:
How long does it take to get three coats of oil-base on your doors? How long from start to ready to hang?

From contributor J:
To contributor G: I don't do three coats very often because it usually isn't necessary, and also because of the time involved. I live in a very humid part of the country, so it can take a while. Quick-dry Alkyd primer has worked well for me, because it's almost always ready to sand the next day, and lots of times late in the afternoon if I spray it on early.

The paint, however, is a horse of a different color (pun intended). I've waited as long as three days between coats (once four days) due to the previous coat still being too tacky to sand.

To answer your question, it can take a full week to get the final coat on whatever I'm spraying, depending on weather, time of year, etc. And, even when the final coat is dry to touch, it won't be fully cured for a few days, so extreme care must be used in handling. A friend of mine won't even touch his stuff till seven days after the final coat.

I've played around a little with Japan drier, but have not been too impressed. It seems like if you add enough drier to make a difference it tints light colored paints. I should also say that I'm a small operation, in fact a one-man band, so what works for me might not be suitable for high production outfits.

From contributor W:
To contributor J: Have you ever tried white lacquer under-coater? What about pigmented lacquer or conversion varnish? I can't imagine trying to handle freshly painted doors or drawer fronts without having damage. If you were painting a job on site and did not have to handle them much, then I could see it. There are many products out there that could make your life much easier.

We switched to pigmented conversion varnish last year and could not be happier with the results. It is self sealing so there is no primer needed. It dries without blushing in about 30 minutes, even in a humid environment. Once it is cured (about 8 hours, you can stack the parts on a cart without having to worry about them sticking to each other and ruining the finish.

From contributor J:
To contributor W: I'm always ready to make my life easier, and will certainly give the conversion varnish a try. In addition to being humid here, it's also rural, (hour and a half to the nearest town of any size) so you can't find or don't hear about the latest stuff.

One question though, how does the CV do with the MDF fuzz on routed edges, which was the original question? That's why I use the alkyd primer, because the heavy solids it contains it seems to do away with that problem. A friend of mine was using white lacquer as a primer on MDF and had the same problems with fuzz as the original questoiner. The drying-time factor, while still a consideration is not as crucial for me because I can usually work around it, and there are no employees to keep busy.

From contributor W:
You can take the CV and thin it by 20%. Use the thinned material to lightly prime only the machined edges. Then sand with a fine sponge then spray with un-thinned material. When we do MDF it usually takes the prime plus two topcoats. It is much less work than either paint or lacquer. You will be able to spray and assemble in the same day.

From the original questioner:
To contributor J: I've never sprayed pigmented CV, but I've sprayed enough clear CV to know exactly why it's called respiratory failure in a can. Beware of buildup of fumes inside cabinets - especially big ones with doors installed. Here in the hot summers we get fumed quite a bit, even a solid day or three after they've been sprayed. I always wear a respirator for all finishes I spray, but I am especially careful of this.

From contributor M:
Does anyone have any ideas what the long term effects of breathing CV fumes might be?

From contributor W:
Fumes from any kind of finish are bad for you. There are very bad things in the waterborne finishes as well as solvent based. A good spray booth, respirator and suit are a good start.

From the original questioner:
I wouldn't try to spray anything like CV, even if it was a drawer knob, without a respirator. I can't handle it, it burns my nose and throat bad enough even after curing 8 hours.

From contributor M:
I realize CV should be sprayed with a mask and proper booth, which I have. I'm just wondering about the fumes that linger for the next couple days, or the fumes that build up in the van if you're making a long delivery?

From contributor R:
Maybe I am old school but I worked in a shop where all we did was MDF doors. First off you have to get the high density MDF for a really good sanding. Next you never blow it off; you wipe it off with a brush.

I like to use Chemcraft products so I use there precat white primer. It has a good solid content and usually I donít have to thin it down to use it. I use sanding sponges to sand them down. First I use Med to get out the router marks and then I use 3ms extra fine to seal it off. I also sand my doors right before I seal them. I spray on a light coat in the route and around the edges on all my doors. Then I go back over the entire door with a little more than a normal coat.

Let them dry then sand them again with the extra fine pads almost all the way back to the wood. One more coat of primer with a light sand and itís ready to finish. We finish in high gloss and they come out perfect every time. I have used many products in the past and lacquer is by far the fastest.

From the original questioner:
The problem was the spraying equipment. We were not, at the time (a couple of weeks ago) in the finishing business, but since our painter went belly up we decided to do it in-house. We only had half a dozen MDF doors to finish on a snack job and we tried it with a regular cup gun.

The primer had to be thinned too thin for use on the MDF (at least the particular type of primer we had). Now that we have the airless system, we can spray the primer full strength and our MDF looks jam-up, the profiles are smooth as silk. It's all in the tooling.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor E:
Having read all the useful tips I put together my own protocol for finishing MDF raised panels using my background in frugality and "use what you got" approach. Here's what I came up with:

1) After shaping the profile, sand with 220 grit paper, then 400 grit paper, but really lean on the 400 grit until the fibers of the MDF start to whiten and flatten and a little bit of gloss develops. The cut profile should feel almost as smooth as the uncut MDF.

2) Prime using white ceiling paint or wall primer, whichever is on hand. Use a disposable foam brush, being careful not to leave thick streaks, but a fine, even, medium-thick coat.

3) Sand with 220 grit paper, but not too aggressively.

4) Sand with 400 grit paper, and again, lean on it pretty good until you get a somewhat glossy surface.

5) Another thin coat of primer, applied with a foam brush or HVLP sprayer.

6) Lightly sand with 220 only if you see any imperfections. If not, you're ready for the top coat of paint.

Here's what didn't work:

I tried steel wool instead of 400 grit, it only serves to leave grey marks everywhere. I tried only using 220 grit, but that didn't get a glossy surface like the 400 grit.

Letting the primer dry completely before sanding with 400 grit gave an inferior result than sanding right after it lost its tackiness; kind of a nice time saver, too.

In closing, I'd add that this technique is probably better suited to raised panel wainscoting applications that aren't at eye level (like cabinetry doors). This works fine for wainscoting, though, since the finish quality is what you would expect from well-painted trim.

Comment from contributor C:
My technique is to use white latex primer applied with a brush, let dry, then knock down the raised mdf fibers with 220 grit sandpaper foam block, rubbed with a vigorous motion, then sand with 400 grit sandpaper, again using a vigorous motion so that the primer gets a shine to it. After that, two coats of premium top coat with a sprayer. My project consists of raised panel wainscotting for my dining room and it turned out great, almost professional quality, dare I say it. Admittedly, this technique involves a lot of elbow grease, but uses materials found readily.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor D:
I have achieved great results using pigmented conversion varnish. On routed edges, I sand with 320. When I spray the part I will do a light pass on edges. After the light pass I will come back to the edges after it has soaked in. Next I spray the face of the part with one last quick pass on edge where I see it looks like it absorbed more paint. After two hours I can sponge sand my edges with a 3m fine sponge. I sand the face with 320, but could use the sponge. I put on one more coat for top coat and the edges look as good as the face. I use Chemcraft CV. Again, I have achieved great results with this - no sizing and no priming, just two coats of CV and it looks great.