I need advice on finishing MDF

[johnzimmerman]

I'm a relatively experienced woodworker but am not a painter and never had to make MDF look pretty, just shop jigs. My sides are 3/4" bb veneered with mahogany and the top, bottom, baffle and back are MDF I want to paint gloss black. Every time I sprayed primer I would see little pinholes, shallow depressions and faint scratches, so I kept filling sanding and priming and could't get rid of all of them. I think some were too shallow for the various fillers I used to keep put thru sanding. So I got disgusted and stripped off waaay too many layers of primer. So can anyone suggest a process or product that might work?

Thanks JohnZ

[Wolf]

I'm a relatively experienced woodworker but am not a painter and never had to make MDF look pretty, just shop jigs. My sides are 3/4" bb veneered with mahogany and the top, bottom, baffle and back are MDF I want to paint gloss black. Every time I sprayed primer I would see little pinholes, shallow depressions and faint scratches, so I kept filling sanding and priming and could't get rid of all of them. I think some were too shallow for the various fillers I used to keep put thru sanding. So I got disgusted and stripped off waaay too many layers of primer. So can anyone suggest a process or product that might work?

Thanks JohnZ

You could try my method if you want. The only thing left out is to use a rubbing compound after everything is done to bring out the shine in the gloss finish.
http://techtalk.parts-express.com/blog.php?b=22

Your prep is very important, and HIGH-BUILD primer is a must. Krylon will not cut it. Like I say in the writeup, when it's butter-smooth, you are ready for clear-coat, or more layers of paint. It depends on what you want to achieve. I would stop at that point and finish it off if it's me.
Later,
Wolf

[Nowd]

I know what you're talking about. Wolf is right when he mentions using a high build primer, but you can get good results with Krylon if you do the prep work. I don't know what kinds of fillers you've used, but you might want to "skim" the MDF with bondo. Let it dry thoroughly, like maybe overnight. Hit it with an orbital lightly with 150 grit paper until most of the bondo is off. Then block sand the rest by hand with 150 grit. Shoot couple of coats primer and block sand it with 320 grit when completely dry. If you see any imperfections, put some primer over them and block sand again when dry. With some patience, you should end up with a pretty slick surface for your color coat.

[Nowd]

After rereading your post, I should add something. It sounds like your MDF is primed and you just don't like the small pinholes, shallow scratches and other imperfections. Get a tube of the Bondo finishing glaze and use it to fill the small stuff. It sands real well with 320 grit so fill any indentations high and let it dry completely. This should give you good results and let you avoid redoing all the MDF. Depending on what color you'll be using, you may not have to prime over the glaze since it's reddish in color. When using black, I don't bother shooting more primer on the spots that I've used the glaze on.

[dbe]

You have asked a question akin to: "How do you mix a good dry martini?"

The problem that I see in finishing MDF is its' inherent softness and porosity. The system that WORKS FOR ME (YMMV) is to use polyester resin or a thin finishing epoxy on the raw MDF before going to any type of paint. I use West Systems epoxy and mix a batch to brush (foam brush) on the surface in question. I lay on a fairly heavy coat and then get my heat gun out to gently heat up the surface. This will drive some of the air out of the surface of the MDF and lower the viscosity of the epoxy so that it will penetrate. It is very easy to go overboard here with the heat. Heat it just to the point that the resin will not soak into the MDF anymore. Too much heat will set the resin up prematurely, especially polyester. Take paper towels or a sacrificial rag and wipe off the surface until there are no shiny spots left and then walk away and let the resin cure. After the resin is cured, scuff sand the surface with 120 to deglaze it and you are ready for primer.

The surface that you have created is a hardshell skin that resists moisture for those of us that prefer waterborn finishes, too.

Again, this works for me. I know that it seems like a lot of work, but it is nothing compared to endless hours of sanding primer over raw MDF.

The resin system works great to harden and seal exposed edges of MDF, too.

HTH

Dave

[Dirk]

For edges, I've had lots of good luck with a 50/50 mix of water and yellow wood glue. Paint a few coats on, sand it down, another coat or two, sand it again, and go on with your finish. Seals the oh-so-absorbant endgrain really well.

I've noticed that it even seals the face to some extent. For front baffles that I intend to paint black (which is 90 percent of them), I end up doing one coat across the whole front baffle. Helps hide any otherwise-unseen glue buildups from doing the edge.

[tomzarbo]

I agree with Nowd's suggestion of using Bondo spot putty on scratches and pin holes. It is normally used hand in hand with plastic body filler (Bondo) and fills in the scratches and air holes that sometimes happen when body filler is applied. It's the red stuff that comes in a tube, and you apply it with a plastic squeegie. It needs to go on thin and it won't cover large surface areas without cracking.
Wolf also previously suggested using Bondo on the end grain of MDF to seal it up. I tried it last week on a baffle for a subwoofer, and it did the trick. It's slightly labor intensive, though, and if you don't put it on thin, you may end up sanding a new profile on whatever you're working on. The good news is that it dries enough to sand in 20-30 minutes depending on how much hardener you use.
MDF end grain is really a bugger to work with, it seems like endless coats of primer do nothing to seal it.
Please let us know of your progress with whatever method you use. I'm still learning too!

TomZ

[williamrschneider]

No one has mentioned yet what can go UNDER the primer. I use a shellac-based sanding sealer first (brush or sponge-roller applied), then primer, then paint.

I don't have spray equipment, so my efforts are limited to can finishes, but I've been pleased with my projects.

I've attached a picture of the coatings used, and a project that was nearing completion. Smooooth!

[JRT]

...use polyester resin or a thin finishing epoxy on the raw MDF before going to any type of paint...

Do you open/remove the mill-finished surface of the MDF with sandpaper before applying resin, or do you apply resin directly over the mill-finished surface?

[Adrian]

FWIW, I have had great results using wood hardener or sanding sealer, both from Minwax. The sealer gets you a good finish off the bat, for priming, but the hardener penetrates deeper on the "end grain," and minimizes chipping and tear outs. No matter which method I use, I always smooth out the end grain with Bondo. I only use filler on veneer holes or gaps, or on solid wood.

HTH,
Adrian

[raycollins]

I use the same materials and sequence as williamrschneider. The bulls eye sealer is dewaxed shellac and the primer is oil based. Be sure and allow the primer to thoroughly dry before applying your finish coat or it will wrinkle...ask me how I know?!

Ray

[dbe]

Do you open/remove the mill-finished surface of the MDF with sandpaper before applying resin, or do you apply resin directly over the mill-finished surface?

I apply it directly over the mill finish. The heat does the trick for absorption of the resin. Edge grain will absorb a lot of the resin if you want to keep applying it until it just begins to thicken. Remove any extra from the edge grain before it cures, though.

Dave

[terryo]

I use Bondo myself, and with different colored hardeners you can create a varigated multihued finish. My "Bondo Beauties" that were displayed at the recent VSAC used no finish over the bondo. I feel that anything that comes between the viewer and the art object detracts from the "Tactile Essence" of the delicate details and the subtle, muted tones of the MDF/Bondo finish.

However, for those of you that lack the soul of an artist, and would violate the cabinet with paint:

After the gross filling of the "endgrain" of the MDF with Bondo, you can apply a several coats of Varathane to the endgrain and joints. Left a week or more to really set up it's then sanded. Practitioners of the barbaric act of painting claim that this eliminates any seams from showing through on a hi-gloss finish. Evidently, it really seals the joints well as it's then ready for primer and paint. If the top finish is urethane, then many skip the primer and apply several coats of paint directly to the MDF.

Best Regards,
TerryO

[terryo]

I use Bondo myself, and with different colored hardeners you can create a varigated multihued finish. My "Bondo Beauties" that were displayed at the recent VSAC used no finish over the bondo. I feel that anything that comes between the viewer and the art object detracts from the "Tactile Essence" of the delicate details and the subtle, muted tones of the MDF/Bondo finish.

However, for those of you that lack the soul of an artist, and would violate the cabinet with paint:

After the gross filling of the "endgrain" of the MDF with Bondo, you can apply a several coats of Varathane to the endgrain and joints. Left a week or more to really set up it's then sanded. Practitioners of the barbaric act of painting claim that this eliminates any seams from showing through on a hi-gloss finish. Evidently, it really seals the joints well as it's then ready for primer and paint. If the top finish is urethane, then many skip the primer and apply several coats of paint directly to the MDF.

Best Regards,
TerryO

[Wolf]

I know what you're talking about. Wolf is right when he mentions using a high build primer, but you can get good results with Krylon if you do the prep work. I don't know what kinds of fillers you've used, but you might want to "skim" the MDF with bondo. Let it dry thoroughly, like maybe overnight. Hit it with an orbital lightly with 150 grit paper until most of the bondo is off. Then block sand the rest by hand with 150 grit. Shoot couple of coats primer and block sand it with 320 grit when completely dry. If you see any imperfections, put some primer over them and block sand again when dry. With some patience, you should end up with a pretty slick surface for your color coat.

I think you misunderstood me, Del.
Krylon paint = fine.
Krylon Primer = Ick!

It's just too thin to do any good. It's like thinned paint, that has been thinned too far. I don't use Krylon for the Prep.
Later,
Wolf

[bill]

Though I don't normally do gloss finishes, I have to second the Zinsser dewaxed shellac sanding sealer. I find it does the job as a base coat under primer. On MDF endgrain I slather on lots of sealer, then sand to 200 grit. I commonly use 1/2" roundovers, and my trick is to glue a piece of sandpaper to the inside of a 3/4" x 3/4" cove moulding, which has a 1/2" cove. By using this to sand the roundovers I preserve their profiles.

After sealing the endgrain, I apply at least two coats of sealer all over, padding it on with a scrap of acoustic foam wrapped in several layers of old T-shirt. Keep a wet edge. Then I sand to 200 grit, spray a coat of finish, sand again and then spray and sand using progressively finer grits. For semi-gloss, I stop at 400 grit, then spray a final coat.

When buying sanding sealer, be sure to get the Zinsser. A number of other companies sell a sanding sealer that's actually polyurethane. Dewaxed shellac will stick to almost anything, and I don't know of a finish that won't stick to it. That's not the case with poly.

HTH,
Bill

[johnzimmerman]

I want to thank everybody for their responses, now I just have to choose my poison. I actually had some shellac and thought to use it but was afraid it was too old. On my way home from work today I got some fresh shellac, high build auto primer, Bondo Body filler and spot filler. I imagine Lou Corragio is rolling his eyes about now, I should have been done with this weeks ago and here I am nearly starting over. I do have some excuses though, business travel and domestic responsibilities do have a way of interfering with the serious business of speaker building.

I will document and post the transformation of the chrysalis to the butterfly as I progress.

Thanks again,

JohnZ

[Nowd]

I think you misunderstood me, Del.
Krylon paint = fine.
Krylon Primer = Ick!

It's just too thin to do any good. It's like thinned paint, that has been thinned too far. I don't use Krylon for the Prep.
Later,
Wolf

I don't know Wolf. It's not the best building stuff, but I've used it in the past and got pretty good results. Of course now that I have a cabinet shop, I haven't used Krylon in a while.

[jonpike]

I want to thank everybody for their responses, now I just have to choose my poison. I actually had some shellac and thought to use it but was afraid it was too old. On my way home from work today I got some fresh shellac, high build auto primer, Bondo Body filler and spot filler.

I like the Shellac as well. A note, the Seal Coat stuff is a special dewaxed shellac, and a bit thinner than the 3lb cut of normal. I'd reccomend cutting it further with alcohol, say to 1-1.5lb cut for better penetration, and doing 2-3 coats, then go from there. The dewaxed variety is preferred due to it's even lower likelyhood to interfere with other coatings. And yes, shellac in general has a limited shelf life, where it eventually won''t fully dry. Good to have fresh if it's more than a couple years old.

Another note... sounds like your MDF might have been rather soft.. easily dinged up, and/or just kinda porous. I've noticed a difference in what the big box hardware stores tend to get, vs a decent contractor supply place or woodworking store. There is higher and lower quality MDF. Might be an idea to find a better source in your area for future projects..


Hope that helps...

[dlneubec]

Yup, thats what I did on my HOSS project also.