Help - MDF Edge Smoothing

21 Jun 2007
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United Kingdom
i am sticking sheets of moisture resistant mdf together. i plan on gluing (polyurethane glue) then sanding with a high grit paper (silicon carbide, 500 grit).

it needs to be prepared for lacquering and so the join needs to be as smooth as possible.

does this sound like a good preparation? or is there something else i need to do?


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lukecarson said:
I am sticking sheets of moisture resistant MDF together. I plan on gluing (polyurethane glue) then sanding with a high grit paper (silicon carbide, 500 grit).
I'd say you need a bit more than that......... Firstly PU has a tendency to expand unless it's properly cramped. It's also expensive, but if that's what you've got just be careful to wear gloves and don't get it on your hands as your skin will go black.... Personally I'd use a good exterior grade PVA

The edges of MDF will need to be sealed as they absorb moisture like blotting paper. There are several ways to do this, including PVA glue diluted with water (5 glue to 5 water), MDF sealant or drywall jointing compound spread thinly on the edges. Sand-off with something like 120/150 grit to start

MDF often has inconsistent surface absorption and may show signs of hydraulic oil contamination from the presses used to make it so it's best to seal all the surfaces. I'd also suggest undercoating the material in the same way you'd undercoat a car body (high build undercoat), then working up through the grits from 120 to 500 to get the dead smooth substrate for your lacquer work. Don't miss too many grits (i.e. go P120 - P180 - P320 -P400 - P500) as you'll only end up having to do much more work to get a smooth surface - or finding that the sanding scratches telegraph through the top surface. With lacquer work every imperfection will shout out its' presence, so clean off the surfaces between grits with a clean, well-washed rag damped with white spirits.

thanks scrit

about the undercoating what would you recommend? i will not be doing the lacquering myself but i will need to prep it. do i apply this before or after the sealant?

to seal i was planning on using rustin's water based mdf sealand (is this better than the pva solution?)

also on the glue, does pva not soak into the mdf more and so increase the chances of movement?

cheers again

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Seal first and flat back then undercoat/prime (preferably sprayed). If you are using a water-based sealant, such as Rustins (and BTW most paint companies such as Johnstone and ICI Dulux supply MDF sealants as well) then I'd seriously suggest two coats for the surface with at least 3 coats for the edges (seal, dry, flat back, repeat.....) because water-based products do tend to raise the grain. That will seal the surface and make it impervious to water, but it still won't be perfect. You'll need to confirm that your sealant is compatible with the undercoat/primer you choose as well - an acrylic sealant might not work at all well with that AC lacquer and its' compatible primer.

The undercoat/primer also needs to be compatible with the top coat you are using, so if you are using something like Sikkens AC then you'll probably need the Sikkens AC undercoat for that. Ask the guy who's spraying it what he'd recommend - or better still get him to blast you a couple of thin coats of primer before he does the top coat. The reason for using an undercoat/primer is to hide any minor surface imperfections in the MDF (they do occur) and to give a surface of consistent tone. If you spray straight onto the sealed MDF the green colour at the edges can sometimes show through lighter top coats as a dirty line.

lukecarson said:
Also on the glue, does PVA not soak into the MDF more and so increase the chances of movement?
I've manufactured dozens of batches of furniture from MDF (a standard method of producing dowelled carcassing in the industry) and not experienced any problems as any "swelling" is for all intents and purposes inside and invisible - PU, on the other hand, tends to foam out and that can become a major headache.

Good luck!


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