What is the best way to paint mdf?

[jhollander]

Re: What is the best way to paint mdf?

Bill, a bit off topic but do you know why the LX521 does not use recessed drivers and a round over on the head piece?

What's the finish on the cherry?

[mattsk8]

Re: What is the best way to paint mdf?

I use a similar method to Lunch's. I thin Titebond original (red label) with water to basically get a latex paint consistency, then I brush that over the endgrain, generally about 3 coats. A 20oz pop bottle cut off works good for mixing the glue with water. I use the cheapest brush I can find, you'll be pulling brush hairs out as your painting the glue on, but I get boxes of 50 brushes from HF for around $10. The woodglue shrinks everything and seals it so the endgrain doesn't "peek through" later on in your project's life.

After this, I let it dry for a day then bondo the endgrain. I'll let the sanded bondo dry for a day as well so it shrinks, then do my second coat of bondo over that. Once this is all done, I spray the whole project with about 2 or 3 coats of filler primer (definitely use filler primer, not regular primer). After that, sand with 400 grit paper to silky smooth, then spray using whatever you'd like. You can get a great finish with spray cans, its just a LOT faster if you use a HVLP sprayer.

All that said, its very tough to hide endgrain, I always build my projects with this in mind and put the endgrain in the least conspicuous area. Also, if you make your endgrain 1/32" short you can fill that with the bondo, and its less likely to peek through later. Dados also help, because then you can make the endgrain 1/2" or even 1/4" depending on how deep you go. I know it sounds like a lot of work, but its the only way I've been able to hide endgrain.

[bill poster]

Re: What is the best way to paint mdf?

a shellac sand seal or diluted pva then prime and paint

[williamrschneider]

Re: What is the best way to paint mdf?

Bill, a bit off topic but do you know why the LX521 does not use recessed drivers and a round over on the head piece?

What's the finish on the cherry?

The finish on the cherry was General Finishes Arm-R-Seal wiped on, not brushed. It's easy to get a very smooth surface with it. Bob recommends micro-fiber towels for wiping, but I've used T-shirt pieces and some industrial wipes in the past too.

I'm not sure why the larger drivers are not recessed, and there's even a warning against doing it. Perhaps Linkwitz didn't worry about diffraction at low frequencies and took measurements with the current mounting. The tweeters apparently don't require it because they have a flange that tapers to a knife-edge to smoothly meet their sub-baffle. The bottom line is that I'm guessing, and that I don't really know why.

[dlr]

Re: What is the best way to paint mdf?

I'm not sure why the larger drivers are not recessed, and there's even a warning against doing it. Perhaps Linkwitz didn't worry about diffraction at low frequencies and took measurements with the current mounting.

I have never recessed a woofer. The benefit is purely cosmetic.

dlr

[Jeff B.]

Re: What is the best way to paint mdf?

The finish on the cherry was General Finishes Arm-R-Seal wiped on, not brushed. It's easy to get a very smooth surface with it. Bob recommends micro-fiber towels for wiping, but I've used T-shirt pieces and some industrial wipes in the past too.

I'm not sure why the larger drivers are not recessed, and there's even a warning against doing it. Perhaps Linkwitz didn't worry about diffraction at low frequencies and took measurements with the current mounting. The tweeters apparently don't require it because they have a flange that tapers to a knife-edge to smoothly meet their sub-baffle. The bottom line is that I'm guessing, and that I don't really know why.

Woofer mounting will play no real role at low frequencies. Even at midrange frequencies the wavelengths are long enough that it's primarily just the baffle width that gets involved.

[Jeff B.]

Re: What is the best way to paint mdf?

I have never recessed a woofer. The benefit is purely cosmetic.

dlr

You do like to place a big piece of felt between the woofer and tweeter though. A think flange might affect tweeter diffraction with some tweeters on an untreated baffle.

[dlr]

Re: What is the best way to paint mdf?

You do like to place a big piece of felt between the woofer and tweeter though. A think flange might affect tweeter diffraction with some tweeters on an untreated baffle.

The flange has an influence, but it's negligible in my testing. The resulting flange geometry makes it more highly distributed in time than does most baffle edges and the angle with regard to the listener probably reduces the impact further.

I do (always) place felt in between a tweeter and any adjacent driver (midrange or woofer) due to the diffraction associated with the extreme depression and larger surface area of the diaphragm. The flange diffraction change is largely incidental. If you surface or flush mount a woofer by a tweeter, you'll likely not see a lot of difference in the diffraction signature on any given axis.

This was made more apparent with my 2-way using an Accuton mid-woofer. The latter showed more diffraction than any other similar sized driver I've tested. That's probably due to the nearly purely concave shape (no vent cover) and the material. There's so little damping in the Accuton diaphragm vs. others. This latter is more conjecture, but the stronger diffraction signature was not. It's easy to test. Measure a raw tweeter that's mounted by the woofer. Then cover the woofer with a good, flat material and measure the tweeter raw again. The difference is uniquely the change in diffraction. Normalize the two and you have the pure diffraction signature of the woofer to the tweeter response If the woofer is not recessed, you'll still have some residual woofer flange diffraction. Then add felt. This should remove almost all woofer diffraction at that point.

BTW, if I had a smallish midrange (4" or smaller probably) close to a woofer in a 3way, I suspect that I would place felt between those as well, though the change would almost surely be much smaller.

dlr

[upsman]

Re: What is the best way to paint mdf?

As far as exposed edges on MDF go here is a possibility

http://www.mlcswoodworking.com/shopsite_sc/store/html/smarthtml/pages/bit_rabbeting_miter_joint.html

[fastbike1]

Re: What is the best way to paint mdf?

Sadly, a lot harder to get edges lined up than it seems. I'm still having trouble after 4 or 5 tries. Viewed the video several times.

As far as exposed edges on MDF go here is a possibility [ATTACH=CONFIG]48765[/ATTACH]

http://www.mlcswoodworking.com/shops...ter_joint.html

[upsman]

Re: What is the best way to paint mdf?

Sadly, a lot harder to get edges lined up than it seems. I'm still having trouble after 4 or 5 tries. Viewed the video several times.

Fastbike, Would it be possible to edge band the exposed edge of the MDF before you glue up some ****joints with some birch or closed grain wood to eliminate the exposed MDF edge..........you could used some decent thickness on the edge band material (1/16-1/8)......at least on the sides your going to see, on the hidden sides you could probably pass if you wanted to save a little effort.

[bobbarkto]

Re: What is the best way to paint mdf?

That won't hide the visibility of a joint line or seam.
It will change the appearance somewhat, but won't stop it.

As an aid to sealing the edge to stop excess finish absorption it seems a bit overkill. There are numerous easy ways to accomplish that task.

Any form of miter joint will be effective but miters aren't always an option. Take a curved cabinet for example. Or where other design elements preclude them or make them too difficult to cut/assemble.

Fastbike, Would it be possible to edge band the exposed edge of the MDF before you glue up some ****joints with some birch or closed grain wood to eliminate the exposed MDF edge..........you could used some decent thickness on the edge band material (1/16-1/8)......at least on the sides your going to see, on the hidden sides you could probably pass if you wanted to save a little effort.

[fastbike1]

Re: What is the best way to paint mdf?

I never use mdf but it's certainly possible to edge band. I have often used edge banding on plywood boxes. My point was strictly that that router bit was as difficult to setup as the ones for hardwood that make a spline on the center. I still haven't gotten it setup satisfactorily after several tries. It's not a make or break for me because I never stain/poly plywood without veneer.

Fastbike, Would it be possible to edge band the exposed edge of the MDF before you glue up some ****joints with some birch or closed grain wood to eliminate the exposed MDF edge..........you could used some decent thickness on the edge band material (1/16-1/8)......at least on the sides your going to see, on the hidden sides you could probably pass if you wanted to save a little effort.

[Carbon13]

Re: What is the best way to paint mdf?

Lots of talk regarding preping MDF for painting, how about the final finish technique? After clear coat, sand and polish? What are the favorite techniques on the final steps?

[bullittstang]

Re: What is the best way to paint mdf?

Lots of talk regarding preping MDF for painting, how about the final finish technique? After clear coat, sand and polish? What are the favorite techniques on the final steps?

I tend to do the long and time-consuming way mainly because that's the only way that has worked for me to my satisfaction.

I use the 50/50 glue mixture, sand, then Filler-Primer, sand until smooth, then paint (usually with RustOleum, but have recently tried the HVLP "much better and faster results") the base coat. Usually takes 3-4 coats with sanding after the 2nd coat to 400, this is where I differ from some other techniques, after the final medium-heavy base coat I wait 24-48 hours and then sand up to 800 wet/dry on that final base coat.

Once I get the surface buttery smooth, then I go with 2-3 light coats of a clear gloss (again RustOleum), let dry for 24 hours and sand up to 1500 wet/dry making sure all runs, orange peel and imperfections are gone.

Last step is 2 final medium-heavy coats of clear gloss again. Let this dry for 48 hours so it's nice and hard.

Now the fun part, sand with 800 wet, 1500 wet, 2500 wet, then move to Cutting Compound (Medium), then Polish and then finally Cleaner/Wax (I use Meguiars, but any high-quality product will give good results, and it makes a big difference what you use) I also suggest you buy a $40 electric buffer, otherwise these sanding/polishing steps will take a full day, rather than a couple hours.

Once you are done, be sure and let it dry for at least a week. the paint will still be soft, so any pressure (sitting on a terry cloth, or anything that's not smooth) will leave an imprint. I know this from experience, it sucks to have it happen after 3 days of painting/sanding/polishing!