What is the best way to paint mdf?

[Pete00t]

Re: What is the best way to paint mdf?

A cabinet maker with 20 years experience told me that he uses auto products for MDF.

does he use epoxy primer or regular automotive primer?

[gowa]

Re: What is the best way to paint mdf?

He didn't mention epoxy.

[jonpike]

Re: What is the best way to paint mdf?

Well, I'll agree with you on this one.
Sanding shellac is a total pain, it was hard work. Maybe next time I'll try thinning the shellac first before applying it.

It works better a couple of ways (soaks in more, doesn't build an unnecessary layer on surface as much) when thinned out. Dries faster, too.

I've have good results with Zinnser Seal Coat cut 2:1 or 1:1 with alcohol. That would take the "2lb cut" Seal Coat down to a "1/2 lb cut" or a "1lb cut". (cut = ancient measuring method of lbs solid Shellac per gallon) The Seal Coat product is a fair amount cheaper than the "regular" shellacs. Dewaxed too, which makes it compatible with nearly any other finish.

As Bob says, it should dry fast, and be very hard. Sands smoother, cleaner and neater than bare MDF, with a white powder. I've not had any gumming with what I've used. The classic test is to pour a tiny bit into/onto a glass dish or surface, if overnight you can still dent it with your thumbnail, it's probably going bad.

If your local big box has only ancient cans, try a woodworking place. If you really want it fresh, you can get the raw resin and dissolve up a batch as you need it.

I'd buy the smaller cans first, till you figure out how old of a date code is safe...

Happy Sanding!

[jemuhammad]

Re: What is the best way to paint mdf?

I've tried a host of things, always looking for the simplest solution. I can definitely get nice results with a rattle can, but prep work is a must. I have learned that you HAVE to seal the endgrain of MDF somehow.

3. The simplest, most reliable method I've found to paint MDF is to thin wood glue with water until it's about the consistency of wet paint, then brush it on the entire cabinet with a foam brush. When that's dry, I sand it down to 220 grit sandpaper, apply another coat of water-thinned wood glue to the whole cabinet, sand that down to 220. After those two coats, the surface is hard, uniform, and ready for action. (It looks like crap, though.) I apply the rattle can of my choice.

Greetings all,
I wish I had found this forum before I started. Here is where I'm at:
1) I designed and built a speaker box made of 1/2" MDF;
2) On the advice of Home Depot, I bought wood filler, fine and medium sandpaper, Kilz sealer and Rustoleum primer & paint in one;
3) I filled and sanded the box to what I thought was good;
4) I sprayed on the Kilz sealer;
5) A few days later I sprayed the Rustoleum primer & paint in one, waiting 48 hours between coats;

I then noticed that the paint never got thick enough to hide the end grain (I wanted my box to look like Paul's and I thought with 3-5 coats I would get there). Reading this forum I am realizing I may never get there just spray more coats.

On the one hand, I would love to take pictures of my box and show it off (pictures of it)...if I could get it to look like Paul's or Dave's);
On the other hand, no one will probably ever see my box as I am mounting it underneath a center console and my main reason for sealing and painting it was to protect it (esp. being made of MDF) from moisture and/or light spills.

Is there a way to still coat it with something to hide the end grain, get the entire surface consistent and then paint it with my nice hammer texture paint?

Thanks!
-Jason.

[bolland83]

Re: What is the best way to paint mdf?

This is another method that I'm testing, skin the cab with 1/8" MDF. So the theory is that the "end grain" area is now reduced to 1/8" wide at the very edge of the cab, if it does show later it's only 1/8" wide so it will be less noticeable hopefully. I have paint on these (no pics yet) and so far so good, but time will tell if it's worth the extra effort.

[Difvision]

Re: What is the best way to paint mdf?

Holy begeezus... that is a port!

[bobbarkto]

Re: What is the best way to paint mdf?

Zinsser SealCoat. It's dewaxed shellac.

Apply a light coat of shellac on the problem areas. Use a cheap bristle brush or pad applicator that you can make. Sand when dry with very fine paper (320 grit, 220 first if you have too many brush strokes).
Apply another coat as above.
Spray a light coat of primer, let dry.
Spray another coat of primer, sand very lightly, you do not want to cut through the shellac.
Paint.

The pad applicator is just some clean white t-shirt material. Cut a square about 4-6". Cut another piece a little bit larger. Roll the larger piece into a ball, place in the center of the square piece. Draw up the corners of the sqaure, twist until it form a somewhat firm ball. Tie it off with string or a zip tie or wire, whatever.
Thin some shellac 50/50 with alcohol.
Dip half the pad in the thinned shellac until it is almost dripping wet. Apply with a long even stroke. It will go on thin, keep going over it. Move quickly.
If the pad gets too gummy dip in denatured alcohol to dislodge some of the build up.
Should leave a glass like film that needs little or no sanding. Just remeber to keep the pad wet but not dripping wet. Squeeze the pad as you go to release shellac.
You can apply a dozen coats very quickly this way. Will dry very quickly too.

[jhollander]

Re: What is the best way to paint mdf?

French polishing?

[craigk]

Re: What is the best way to paint mdf?

French polishing?

john, pretty sure that is something else all together. :D

[bobbarkto]

Re: What is the best way to paint mdf?

Almost. It's called padding by some.
A few shortcuts in the process and the technique is not as finessed as a true french polish. No oil is used either since it can interfere with paint adhesion.

I often use this method on raw mdf edges. It's quick and easy.
On raw mdf sand smooth to 220-320 grit. Brush on a heavy coat of shellac from the can. I will sand this with ~320 wet/dry paper while the shellac is still somewhat wet (it soaks in so much that the first application will take quite a while to dry).
The dust mixes with the damp shellac and makes a filler as you sand.
A wipe or two with the pad after it's dry and it's fully sealed and like glass.

[bolland83]

Re: What is the best way to paint mdf?

Almost. It's called padding by some.
A few shortcuts in the process and the technique is not as finessed as a true french polish. No oil is used either since it can interfere with paint adhesion.

I often use this method on raw mdf edges. It's quick and easy.
On raw mdf sand smooth to 220-320 grit. Brush on a heavy coat of shellac from the can. I will sand this with ~320 wet/dry paper while the shellac is still somewhat wet (it soaks in so much that the first application will take quite a while to dry).
The dust mixes with the damp shellac and makes a filler as you sand.
A wipe or two with the pad after it's dry and it's fully sealed and like glass.

I've done something similar using wood glue before, also works pretty good. Gunks up the sandpaper pretty quick though, but 220 grit on an orbital sander makes quick work of it.

[dlr]

Re: What is the best way to paint mdf?

I often use this method on raw mdf edges. It's quick and easy.
On raw mdf sand smooth to 220-320 grit. Brush on a heavy coat of shellac from the can. I will sand this with ~320 wet/dry paper while the shellac is still somewhat wet (it soaks in so much that the first application will take quite a while to dry).
The dust mixes with the damp shellac and makes a filler as you sand.
A wipe or two with the pad after it's dry and it's fully sealed and like glass.

My dipole has no box (of course), so the rear of the baffle is open to view. I also have now rounded two side edges on the mid/tweeter baffle. Sanding the roundover will probably mean doing that by hand, but the real problem is what to do with the driver cutouts. Those are chamfered. IIRC there is a spray can version of Zinser shellac (or some company shellac). What is the opinion on these products? It might make working the chamfer area easier. I'm also curious about the roundover. Would a spray version go on easier as well or is that just a cheap-n-dirty shortcut that will look like a cheap-n-dirty shortcut?

dlr

[emilime75]

Re: What is the best way to paint mdf?

Not sure if it's been mentioned yet as I haven't read through the entire thread but, i used Zinsser shellac primer on mdf recently and it did a great job of sealing up the mdf end grain. My process was to sand edges as smooth as possible before applying primern prime, let dry, sand again, prime again, sand again...paint. My local Depot didn't carry the shellac, Lowes did.

[bobbarkto]

Re: What is the best way to paint mdf?

If it is the only finish you will use then it could be an excellent choice.
I would not wet sand using the method I described above as it leaves a less than uniform color appearance. However, it is very effective at sealing for opaque top coats.

The spray version is not dewaxed. It can cause adhesion problems. Most coatings will not adhere to it.
Many coats of the spray will be needed on mdf edges. The material is thin.

I see no issue with applying SealCoat by hand to roundovers or chamfers or cutouts.
The first brushed on coat or two soak in and leave no film, no ridges, drips, bumps.
Wiping or padding the next coat or two should be even easier.

If a smooth surface is desired it will need sanding no matter what product is used.

My dipole has no box (of course), so the rear of the baffle is open to view. I also have now rounded two side edges on the mid/tweeter baffle. Sanding the roundover will probably mean doing that by hand, but the real problem is what to do with the driver cutouts. Those are chamfered. IIRC there is a spray can version of Zinser shellac (or some company shellac). What is the opinion on these products? It might make working the chamfer area easier. I'm also curious about the roundover. Would a spray version go on easier as well or is that just a cheap-n-dirty shortcut that will look like a cheap-n-dirty shortcut?

dlr

[williamrschneider]

Re: What is the best way to paint mdf?

I use a method similar to Bob's on MDF. Hand wipe Zinsser Seal Coat dewaxed shellac, three coats with sanding between, then paint. The first coat of shellac always gets completely absorbed by the MDF, especially the edges. No brush marks will develop at all. I give it a second coat of shellac after some light hand sanding, and sand it again. One more coat with some very light sanding and I'm ready for paint.

I always apply it using a piece of old T-shirt material.

Here's a photo showing the chamfered edge of a piece of 1-1/8" thick MDF stair tread from Lowe's done in this manner. The unfastened connector bolt is sitting on its surface...



It doesn't take much work to get it ready for paint using shellac. At least for me.