Prepping edges of MDF for paint

Hi Guys,

Hope i've posted this in the right spot.

Quick question, i've knocked up a basic toybox for my son out of MDF. I want to paint it but know i need to seal the edges or it will soak up the paint and swell...

All resources on Google i've come across say to use "Drywall Putty", is that essentially Spackafiller type stuff?

I don't need a high end finish as this box will get absolutely destroyed in about a year if i know my son (or he's anything like i was as a child). I just need to seal the edges before painting (oil based paint obviously).

Cheers
Dibs

Dibs,

You can use Plaster top coat or patching plaster. Cheapest way would be to buy patching plaster in dry powdered form and mix up a small batch with water until it is the consistency of toothpaste. Apply it along the edges and sand flat when dry. This will stop the paint from soaking in as the plaster fills up the pores in the mdf edge.

Spakfiller would work if you already have some but if not the cheapest plaster patch/ filler will suffice.

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Originally Posted by skot

Dibs,

You can use Plaster top coat or patching plaster. Cheapest way would be to buy patching plaster in dry powdered form and mix up a small batch with water until it is the consistency of toothpaste. Apply it along the edges and sand flat when dry. This will stop the paint from soaking in as the plaster fills up the pores in the mdf edge.

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Sweet as, thanks for the advice

Just use undercoat before you paint, that's what it's for.

Trouble is you need to apply about 3 coats to fill the edges. (spraying) DAMHIKT

1 coat of non-spray paint has done it for me

I have used undercoat on MDF in the past. I painted the edge using a 10mm brush and once that dried I did the rest of the shelf. It was always going to be a temporary fix as years later the cupboard was dismantled and replaced.

I painted some MDF doors in 2pack and used a auto Primer surfacer [$14}in a spray can on the routed edges, about 3coats needed but you can sand been coats in about 20 minutes to get rid of the fur.
Also tried Sanding Sealer years ago then sprayed undercoat then acrylic. Primer surfacer WINS.

Any oil based primer is the go... I have used sanding sealer before.

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Originally Posted by elanjacobs

1 coat of non-spray paint has done it for me

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200 edges at approx 575 mm each is a bit to much for me by brush, plus I do 100 each of naughts and crosses at 50 diameter and square, that's a lot of edges.
Kryn

On my model making - I use a thin smear of bodyfiller and then sand smooth - works great

Regards

Keith

Gyprock Premix Plaster Filler Jointmaster --> https://www.bunnings.com.au/gyprock-...aster_p0730841

I produce a huge number of MDF items every day (its my main business for the moment) for shipping to two finish painters in Melbourne to squirt before delivery.

Everything is sanded with 120, then this with a spatula for flat areas (sides).... I cheat and use a wide chisel. A finger for complex edges. Another sand with 180 and its finished. Another coat if I missed a bit or there is a mistake to bog up.

Edges are perfect.

Timbermate will work as well. I do like the Timbermate, its easy to manage and uses just water (as does the Gyprock). Car bog works very well too, but dont forget to sand while its still "green". Bog is good for fixing up dinks and mistakes as it has excellent build, mould-ability and shaping :) Timbermate and Premix do not - they are just for finish filling.

With both, you still need to follow good finish paint protocol, just as with any panel finish. I use Haymes tricoat then finish in their enamels. Ive also used enamel spray packs and auto finishes with **perfect** results on MDF.

I used to just shoot Haymes Tricoat as a duster, sand, add a light wet coat, sand and then a final full wet coat, but with the premix I can just jump straight to the wet coat.

Dibbers,

Sanding...... I did the same, made a toy box out of MDF, I sanded every edge with an orbital sander to 400grit (wet/dry -dry obviously), it finishes the edges off like he face, smooth and ops absorbtion. I still under coated it though i jut used white ceiling paint with microban to stop mold as MDF is worse than a sponge.

it feels as though the heat generated through sanding melts the glue (what ever the agent is that holds it all together) and seals the edge. try it out on a test piece, you'll see what I mean.

Richard

Great thread... so much useful info.

Hopefully this isn't a hijack but I have also recently made an MDF toybox for my son and sealed the edges and holes with gyprock topcoat (so glad to see that wasn't a mistake!).
But (unfortunately?) after I sanded I threw on a couple of coats of acrylic paint I had lying around... it looks good but it's not even remotely robust and if I wipe it down with a cloth the paint comes off.
I was thinking of putting on a protective coat like Cabothane clear to give it a bit more robustness/moisture resistance but I'd love to hear some recommendations.

Cheers,
TC

I recall being advised to use watered down PVA glue to seal the edges but I have never tried it so can't comment on the result.

I have just recently made Mdf doors for an entertainment unit. All edges were sanded to 800 grit which left them pretty shiny. I then used a 2k primer followed by a light sand and then 2k topcoat. Edges come up perfect, just need more practice at reducing orange peel on the faces

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Originally Posted by KBs PensNmore

200 edges at approx 575 mm each is a bit to much for me by brush, plus I do 100 each of naughts and crosses at 50 diameter and square, that's a lot of edges.
Kryn

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Do you have a spray gun. If so you could use spray putty. You would need a gun with a minimum 1.8mm setup. You would need several coats but it’s almost instant dry.

When I do edges of things, I put tape along one edge, then angle the gun down so the lower edge doesn’t need taping.

Quickest way to do that quantity that I can think of.

It also comes in rattle cans but way to expensive that way.