Priming MDF boxes

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I've read quite a few threads on this site (and others) about sealing and painting MDF (some great stuff here by ShinOBIWAN BTW). I've been looking to find a way to seal MDF well enough that I can laminate sheets of MDF together, and get a smooth finish, without the edges showing through. The following is the results of some experimenting I've been doing, and hope that anyone with thoughts/experience will post with their knowledge.

I took some circles (approx 3" diameter) cut from 3/4" MDF and glued stacks of five together to make 3" diameter 3 3/4" tall cylinders. I then covered the stacks with the following products:

* Screwfix Direct MDF primer (water based)
* West System 2 part epoxy
* Shellac Sanding sealers (can't remember the brand)
* High solids two pack automotive primer

I've never had much luck with water based MDF primer - mixing water with MDF never seems like a good idea anyway. It's quite thick (like cream) and can't be sprayed. Painting it leaves plenty of brush marks, which need to be sanded out.

It wet sands reasonably well, but you usually end up sanding through to the MDF, so you often need several coats.

The epoxy is impressive. It needs quite a bit of sanding after painting, but it will take it.

The shellac is surprisingly good. It's very liquid, easy to brush on, doesn't leave many marks and is easy to sand. You do need lots of coats as it doesn't build up very thick.

Finally, the two pack primer covers very well (spray or brush) and wet sands wonderfully. The downside is that it's dangerous to use without proper breathing equipment.

After priming and sanding, I sprayed a couple of coats of cellulose paint on all the stacks and left them for a few days to check the results:

* MDF primer - joins between circles very obvious
* Epoxy - joins almost invisible
* Shellac - joins just visible (very fine lines)
* High solids two pack automotive primer - joins almost invisible

I didn't coat the base of the stacks (just the top and sides) so the moisture content of the MDF could change over time. I'm going to leave the stacks for a while longer to see if the lines become more obvious.

Given the dangers of the high solids primer, I'm thinking of sealing bits you won't see (i.e. the insides of speaker boxes) with shellac, and coating the outsides with epoxy.
 
I've been sealing the cut edges with drywall compound, as I read somewhere, and useing an oil primer. (using a foam roller for the primer coats seems to be as smooth a job I can get without a spray booth)

The drywall compound sealed the edges as good as PVA, but sands much nicer.
 
I would recommend treating the inside the same as outside.
If you leave an avenue for moisture migration, it will be found.

Watch that shellac sample over time. The penetrating properties of shellac help, but it is still permeable.

With epoxy it is important to apply two coats. And equally important that the first coat not be sanded. The second coat of epoxy should build a smooth base for subsequent topcoats. If there is some irregulaity in the second epoxy coat you can spray a surfacing primer over it that is easy to sand.

With epoxies it is important to observe the recoat "window" or you will have to scuff sand for good intercoat adhesion.
 
IMO, the best result I got is with Pro paint stuff. Try to buy a can of primer builder from an auto parts store. This is quite commun stuff. This is like fine putty but in a spray can. This take many coats, maybe 4 or 5, but when you sand, you must be careful not to remove it. Do the same for the paint and buy also the clear coat from the same store.
 
Sploo

Shellac makes a good filler/sanding sealer. It is my usual choice for wood or mdf. The only problems with shellac as far as I know are;
1) it is only a surface coating - it does not penetrate and therefore does not make it completely sealed
2) It is important not to put a coating onto it that sets more slowly than shellac otherwise it can cause ripples - although this can also occur with other primers also.

I am not sure how you plan to finish the outside eventually but a thickotropic emulsion paint can make a good primer/sealer.

Don
 
Great work Sploo. I have a pair of speakers I need to seal and the Wood Hardener products are just too expensive, so I have been looking into epoxy. Your findings have given me the impetus to try it.

I need the cabinets to be very tough as they are for taking to house parties. Do you think the epoxy increases the resistance to damage of MDF (particularly the edges)?
 
454Casull said:
Would you mind trying a solvent-based (oil or alcohol) primer?

I'd be quite happy to give it a go. Could you post the names of some of the products you're thinking of? I might not be able to get them here in the UK, but with brand names I could probably find something similar.

I've also been recommended to look at Alkyd based primers, but it's not something I've investigated yet (don't know anything about them).


ruerose said:
The drywall compound sealed the edges as good as PVA, but sands much nicer.

Interesting. I don't think we use drywall that much in the UK (could be wrong). I don't recall seeing any such sealing products in UK stores.

I have tried PVA to seal MDF edges, but never found it satisfactory (hence I didn't test it this time).


Bob Barkto said:
I would recommend treating the inside the same as outside.
If you leave an avenue for moisture migration, it will be found...

Good points, thanks. I was hoping to not use the epoxy for everything, as it's quite expensive.

The instructions for the West System epoxy are very similar to what you've posted (and what I followed).


ostie01 said:
IMO, the best result I got is with Pro paint stuff. Try to buy a can of primer builder from an auto parts store. This is quite commun stuff. This is like fine putty but in a spray can. This take many coats, maybe 4 or 5, but when you sand, you must be careful not to remove it. Do the same for the paint and buy also the clear coat from the same store.

Sounds a lot like the high solids primers (putty). The problem with cans is that they tend to be very expensive when painting large areas, and it's often harder to find the stuff here in the UK.


AMV8 said:
...I am not sure how you plan to finish the outside eventually but a thickotropic emulsion paint can make a good primer/sealer.

Could you give me some product examples? I suspect they may leave lots of brush marks, thus needing plenty of sanding. Quite happy to try it though.


richie00boy said:
...I need the cabinets to be very tough as they are for taking to house parties. Do you think the epoxy increases the resistance to damage of MDF (particularly the edges)?

Once 'entombed' with epoxy, the object looks and feels more like a rubberised/plastic block. Kinda weird to be honest.

It remained slightly rubbery for a few days, but then set like resin plastic.

It does seem to be pretty tough, but if you painted the box then I suspect it would still take damage as normal. I.e. paint chips.

It's possible the epoxy may hold the corners together (preventing chunks being chipped off) but I don't think it would stop deformation on impact.
 
Drywall = Plasterboard = Gyprock.

The jointing compound comes in plastic tubs. There is base coat, and top coat. You'll want top coat. I reckon you'll find it at B&Q, or if not, 'fine surface' polyfilla is as close as you'll get, and that is what I use for this purpose.

I have found (without resorting to exotic stuff for boats) that if (in your example) you chamfer the edges of each piece to accentuate the join, prior to assembly, you can fill the gap like you would with plasterboard, only without the tape, and you will not see the lines after normal finishing. It is important to increase the size of the join in order to properly conceal the line when finished. A 2mm chamfer is all that is required. You'll need a couple of raised coats of filler, as there will be shrinkage, but you can slap it on nice n thick, as it sands really easily, even by hand.
You'll still need to seal and prime.
I have tried sealing and priming without the chamfer, albeit with less exotic products than west systems epoxy, and have not bettered this method.

Try it on your circles... you'll be impressed!

Regards, Bob
 
bobhayes said:
...you chamfer the edges of each piece to accentuate the join, prior to assembly, you can fill the gap like you would with plasterboard

Interesting. I'm not sure if adding the chamfer will be practical for my ultimate intended application for the lamination technique, but it would be worth trying to see how it goes.

Is there any risk that the finished compound (or the MDF underneath) shrinks further and develops cracks?
 
diyAudio Member
Joined 2004
Hi Sploo, good to see someone else exploring this. Its been a big issue for me and has cost me a lot of time.

The best method I've tried is veneering the edges:

tism37.jpg


tism54.jpg


First off, apologies for these large images. I always try to strike a compromise between size and quality but I just couldn't keep the needed detail with reasonable sizes.

The following are close ups of the metallic silver finish on the baffles, this colour and finish really show every imperfection because the metallic has a texture. If you look at the last shot of these three you will notice a faint dent in the finish, I did this after prepping to paint and it was so small I thought "what the hell". Its very plain to see in that photo, yet still absolutely no lamination joints which show even more plainly than that. I took these today so its been 3-weeks now since finishing them.

tism94.jpg


tism95.jpg


tism96.jpg


Overall the veneer does a brilliant job here and its supremely quick compared to the other methods I've tried. The only thing I've done here is veneer and then prime. No other prep necessary.
 
ShinOBIWAN - Very Very Awesome!!! Looks like a great solution. As with many other diyers, I've been struggling with this issue.
May I ask, what type and thickness of veneer do you recommend for painting?
Will be painting speakers a lacquer gloss "piano black". Using MDF for outer cabinets. Painting in an automotive paint booth pro spary gun.
Also, any special application techniques recommended for the veneer?
Hope you don't mind, Jim
 
strange shape the speakers may be but they do look well done :)
be nice to see a finished picture.

some of the veneer dont seem an easy cut being the shape it is any tips

guessing it was like iron on and where did you buy it from cos i am in the UK as well first time i tried i messed it up ended up just painting it .

and then i done vinyl and bit since so got better but any tips be good :)
 
diyAudio Member
Joined 2004
larksp said:
strange shape the speakers may be but they do look well done :)
be nice to see a finished picture.


They aren't finished yet so nothing to show just now. There's a thread on here that shows the construction progress.

some of the veneer dont seem an easy cut being the shape it is any tips

Not really any tips to give. It looks more complicated than it is.

I used a plain veneer with as little grain pattern as possible, I think it was Birch. Then simply cut roughly to size with sharp scissors, applied contact adhesive to the veneer and cabinet, fixed into place and applied large pressure to make sure it was thoroughly stuck down then simply trimmed with a very sharp scalpel and finally sanded flush.
 
diyAudio Member
Joined 2004
slhijb said:
ShinOBIWAN - Very Very Awesome!!! Looks like a great solution. As with many other diyers, I've been struggling with this issue.
May I ask, what type and thickness of veneer do you recommend for painting?
Will be painting speakers a lacquer gloss "piano black". Using MDF for outer cabinets. Painting in an automotive paint booth pro spary gun.
Also, any special application techniques recommended for the veneer?
Hope you don't mind, Jim

Thanks.

It was the first time I tried this approach so I'm a little new to this myself. After the 3 weeks from initially, if you don't have very faint lines showing then you'll have nothing in the future and this is just that so I'm confident this is the best way I've found so far.

I used what was described as a 0.6mm veneer I measured it and it was 1mm however - quite thick for a veneer. I'm almost sure it was a Birch, actually no it was Birch(just checked). This veneer is good for this application because it has no heavy grain or figuring which cuts down on finishing work further. Its also stable, very flat and easy to trim using just scissors.

I mentioned a bit about the veneer application above. Its really very simple. I was somewhat scared off by the seemingly awkward shapes at first but after doing it, I couldn't believe how easy it was.
 
I have used shellac, first coat thinned with alcohol for better penetration, second coat also thinned but not so thin as the base coat.

One thing I discovered by accident is wood putty thinned with alcohol. I use it to smooth dents etc, but have noticed that a thin mix seems to seal MDF quite well. You might add that to the list of things you are comparing.
 
ShinOBIWAN said:
Hi Sploo, good to see someone else exploring this. Its been a big issue for me and has cost me a lot of time.

Hi Ant, I was hoping you'd reply!

Yea, this MDF stuff is a real PITA to seal. I've just bought some Ronseal wood hardener to use for another round of experiments, though I do notice it states on the back that it's not to be used for sealing and priming. :xeye:

I had considered some form of veneering, though I'm planning on creating some very 'curvy' cabinets, so I was unsure as to how practical that would be. Thought looking at the results you're getting, it might be the only way.

Because I don't think I made a particularly good job of glueing the discs together (just blobs of glue, instead of a nice brushed layer) I want to try using the epoxy again (as my first epoxy test piece still looks pretty good). I'll also test the wood hardener.

BTW Large images are preferred, at least from my POV - it's much better to see the detail than try to work out what's going on in a postage stamp :D

Paul W - wood putty + alcohol is an interesting idea. What products are you using?
 
It is "Elmers" brand wood filler...a yellow/tan paste. In the US, it is available in 1 pint plastic tubs from home improvement stores...you likely have something quite similar in the UK. Generic denatured alcohol is also available in home improvement stores.

Just rubbing a thin mix into end grain cuts seems to seal fairly well. Try it and let me know what you think.
 
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