Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

Hey Dakkanauts -- how do you seal your MDF buildings before painting?

Some time ago, I ordered a load of Game Craft MDF scifi buildings for use in Tomorrow's War with Khurasan Space Demons and Exterminators. which project ended up on the backburner. But with some MDF stuff on the way for other pursuits, I figured I should get some experience.I tried sealing the surfaces with water-diluted MDF but that didn't prevent the PVA from developing a very grainy texture after taking primer and a base coat. I then tried Dullcote (the only clear lacquer product I had at the house) and the MDF seemed to take paint marginally better thereafter.

What with all the love for MDF throughout the hobby these days, I know there has got to be a better way.

Anybody?

Here's a pic of the finished module:

I've had good results just priming with grey automotive primer. I've tried using white automotive primer but the MDF just seemed to drink it all and left a very patchy finish.

Once the grey primer dried it took paint as well as any other surface.

I have no answer for you but I am interested to see what answers you get. I'd like to learn more.

I'm with paen on this one! I don't have a good answer but i will myself be working with MDF shortly and want to achieve a good finish too. some things i've read but haven't tried have been watered PVA glue, priming the surface with a household paint/acrylic sealing with a spray varnish first. although I must say your model looks good!

Big part of why I've steered clear of the stuff. Count me as the third hoping for a good answer! My first (untested, inexperienced) thought would be a gloss varnish since it has such a strong bond, then maybe a matte over it to help the painting.

I used watered down PVA/emulsion mix on mine. I also did this prior to assembly (I hate the smell so wanted it completely sealed!) A bit of expansion was the result!

I did this following the logic of using PVA to seal before paint as we would in construction so as not to loose paint coverage but having spoken to quite a few of the manufacturers about this at Salute almost without fail they used regular spray primers/paints.

2 methods used by woodworkers:
1. Sanding Sealer. Apply, let dry, sand smooth.
2. Glue-water mix...anywhere from 10:1 (glue to water) all the way to 50/50. The key on this is to use the yellow wood glue (it has the viscosity of PVA, but is yellowish and says specifically "wood glue" on the container), not PVA.

If you want a really smooth finish you have to sand it with fine grade paper, then wash.

For modelling, I would prime it with watered PVA glue, however decorator's wood primer would do just as well.

 Kilkrazy wrote:
If you want a really smooth finish you have to sand it with fine grade paper....

This is probably the best answer you're going to receive. I've never messed with MDF as part of a hobbying project, but I work with it all the time as part of (one of) my day job(s). You can prime it and paint it like anything else. But like most fibrous materials, painting is going to raise the fresh/exposed grains. Lightly sanding between coats will get you a very smooth finish. It requires a little bit of touch but is very straightforward and easy to do.

MDF is also a very porous, so you might want to consider an oil based primer for raw material. You can use a clear sealer if you want to, but it's probably not necessary for hobbying applications.

One would think sanding will obliterate detail.

Yes, you would only sand flat surfaces, on an object that small.

Hello,
I use a large amount 3/4" MDF at work and when finishing it, we use a combination of thin super glue on the edges and Automotive primer, as SilverMK2 suggested. Sanding between coats is key. The MDF will absorb the glue and primer. However, when I use thin super glue on the edges, sand it out and coat with automotive primer, it goes a long way towards making a paintable surface. Usually 1 coat of super glue, sand, primer, sand and possibly a second light coat of primer is what we use, prior to final paint.

Hope this was helpful.

How do you apply the super glue?

Sorry for the slow response, I decided to take a few pictures to explain.

I either use the regular glue nozzle, or I use an extra thin applicator tip, pictures #1 and #2.

The thin super glue (CA) will soak into the edge grain of MDF very fast, HDF might take longer and I would consider wiping it off if needed. You only need a thin coat to keep the primer or paint from soaking into the board.

After a quick wipe with a paper towel, to avoid any lingering glue that will clog your sand paper, I use either a homemade sanding block, or folded over wet dry sand paper (220 -400 grit, your choice). Pic #3...

There is usually a thin residue of MDF dust left over from sanding. I recommend coating that again with glue and sanding one more time, pic #4. Any paint or primer applied over that should be fairly smooth, in my experience.

I prefer Thin CA over the Medium, or Gap Filling formulas, as it does not obscure detail as fast and can be wicked up with a paper towel if done quickly.

Generally, I sand the front and back surfaces before primer, but will add thin CA in certain situations, such as when I use a Dremel to rough up the surface. The glue seals the rough surface the same as if it were edge grain.

If I missed a step, or anyone has any questions, I will post back with pics or better descriptions.

Thanks

Thanks for the pics. My question, however, goes to how one would go about applying CA glue to a broader surface -- say for example a wall of a miniature structure.

Manchu,
I suggest sanding and priming for a broad surface.

If you must apply super glue, the thin CA can be poured onto the bottom of an inverted paper or plastic cup, then wicked into a folded paper towel and spread onto the surface of the MDF. I highly recommend rubber gloves because the glue can wick through the towel and onto your hands. You should fold the paper towel into a "brush-like" shape. You can wipe it across the surface, quickly, to seal the MDF. It is a method that uses a lot of glue and stinks, but it is fast.

Be sure to let the paper towel fully harden before throwing it away, it causes the CA to cure very fast and can smoke. Try not to breathe it in and work with a fan if possible.

I attempted to take a one handed video and it proved rather shaky. So here are a few still shots. I will figure out how to post video and try to get something better posted if the still shots are not clear.

It is a potentially messy and pricey method to seal MDF, but it is fast and works well. Usually I prime broad areas of MDF, then sand. But the CA method does work, but depending on the size of the area to be sealed, it can use most, or all of the glue.

Seems a good technique, thanks for posting that!

You wouldn't, he is only using the superglue on the very porous edges. For the flat sides, just sand and prime as normal. Sand lightly between coats.

Manchu,
Happy to spread the knowledge.

Todosi,
I agree, sanding and priming is a much easier method for sealing the flat sides of the board. But the thin glue will soak into the broad, flat sides of MDF and it has its place in my bag of tricks, mainly for sealing edges, that is correct.

I have had some success in sealing with gesso, but have only done a few test pieces.

I too am watching this for new ideas..

You probably already know this but on the off chance you (or maybe just another reader) doesn't, this is the specific superglue he is using. It is water thin and sucks for normal hobby use for for some really specific jobs it's absolutely amazingly perfect. One tiny drop in the corner of a tight-fit join and it will fill itself from end to end. If you're not using it in this method pictured earlier as a wash almost, you will want a needle applicator or it becomes a mess fast.


Anyway, for the PVA glue - i wonder about misting it with a few super thin coats of mostly water with a tiny bit of PVA in it through an airbrush? I've never tried it, the MDF buildings I have were all supposed to be ruined stone buildings so the texture was not a problem for me. I suppose that might cause it to warp though.

I would never run PVA (even watered) through an Airbrush.

Just put the mix into a regular hand pumped spray bottle/diffuser. There cheap as chips and will do the job fine. I'm pretty sure Woodland Scenics sell and empty one but you will as easily find one in supermarkets or drug stores.