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Priming MDF boxes
Priming MDF boxes
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Old 22nd April 2007, 08:44 PM   #1
sploo is offline sploo  United Kingdom
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Default Priming MDF boxes

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.
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Old 22nd April 2007, 10:46 PM   #2
454Casull is offline 454Casull  Canada
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Would you mind trying a solvent-based (oil or alcohol) primer?
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Old 22nd April 2007, 11:14 PM   #3
ruerose is offline ruerose  Canada
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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.
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Old 23rd April 2007, 04:09 AM   #4
Bob Barkto is offline Bob Barkto  United States
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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.
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Old 23rd April 2007, 05:22 AM   #5
ostie01 is offline ostie01  Canada
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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.
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Old 23rd April 2007, 11:28 AM   #6
AMV8 is offline AMV8  United Kingdom
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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
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Old 23rd April 2007, 11:37 AM   #7
richie00boy is offline richie00boy  United Kingdom
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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)?
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Old 23rd April 2007, 01:54 PM   #8
sploo is offline sploo  United Kingdom
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Quote:
Originally posted by 454Casull
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).


Quote:
Originally posted by ruerose
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).


Quote:
Originally posted by Bob Barkto
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).


Quote:
Originally posted by ostie01
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.


Quote:
Originally posted by AMV8
...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.


Quote:
Originally posted by richie00boy
...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.
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Old 23rd April 2007, 02:38 PM   #9
bobhayes is offline bobhayes  Australia
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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
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Old 24th April 2007, 04:27 PM   #10
sploo is offline sploo  United Kingdom
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Quote:
Originally posted by bobhayes
...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?
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