Prepping MDF.

[Peter Hamilton-Scott]

I've got some MDF I want to paint and a couple of things come to mind.

1. Where I've joined some MDF I'd like to try and hide the join so that it does not show when painted. There's not enough of a gap to use wood filler but perhaps there is something I can cover the join with that will do a good job of hiding it?

2. Where I've got visible screws I've countersunk them in below the surface of the MDF. Will wood filler be the best stuff to use to fill them in and hide them when painting them over?

3. I've got some markup lines here and there where I used a fine ballpoint pen. I've sometimes noticed that these lines seem to repel the paint and they continue to show through. Any ideas?

4. Some of the MDF has what I think are grease-like stains on it when I got it from the yard. What's the best way to remove them? I'm not sure if using white spirits, etc is a good idea or not.

Comments appreciated.

 

[mwhafner]

You can use just about any filler that is not water-based on MDF.  I generally use Bondo, but have used vinyl spackle, wood filler, etc with success.  

For cleaning the surface, I would wipe it down with denatured alcohol.  

Make sure you use a good oil-based primer, which should cover your pen marks.  I actually like Kilz primer for MDF.  I will put on a heavy coat, sand thoroughly.  Then I will apply a thinner second coat, sand lightly and then apply paint.  

Depending on project, you may want to seal the edges with shellac.  They will soak up a lot of primer unless sealed.

 

[mastercabman]

When it comes to joints,it is hard to hide them.I usually make sure that i use glue in the joint to seal it.
For screw holes,i use a carpenter putty,it dries very hard and it stays.But you need 2 application.Most of the time when it dries it shrinks a little.and make sure you let it cure.If not it can still shrink and then you need to reapply.
Any small dent/line,you can sand them out with some 150/180 grits,and for the stain,sand them a little,clean them up and use a good sealer/primer and that should do it.

 

[mastercabman]

I also like to add to use a oil based primer like mwhafner suggested.

 

[jonny round boy]

I've got some MDF I want to paint and a couple of things come to mind.

1. Where I've joined some MDF I'd like to try and hide the join so that it does not show when painted. There's not enough of a gap to use wood filler but perhaps there is something I can cover the join with that will do a good job of hiding it?

If you know you're going to fill the joint afterwards, it's best to bevel the edges of the MDF slightly (1-2mm), so when they're joined you end up with a V groove. This then gives something for the filler to key into. Best filler to use is the 2-part type, where you mix the filler with a small amount of catalyst - something like THIS

2. Where I've got visible screws I've countersunk them in below the surface of the MDF. Will wood filler be the best stuff to use to fill them in and hide them when painting them over?

Just use the same filler as above.

3. I've got some markup lines here and there where I used a fine ballpoint pen. I've sometimes noticed that these lines seem to repel the paint and they continue to show through. Any ideas?

Just sand the surface slightly before painting.

4. Some of the MDF has what I think are grease-like stains on it when I got it from the yard. What's the best way to remove them? I'm not sure if using white spirits, etc is a good idea or not.

Again, sanding the surface slightly will take most of it off.

When it comes to painting, there are dedicated MDF primers available. I tend to use one of these. Alternatively, you can paint all the edges with sanding sealer, and then de-nib that when dry & paint with a standard oil-based primer.

 

[AlexThePalex]

Like the guys above said, use a 2 part filler. Make the bevel johnny round boy mentioned. This will give you more area for the filler to attach to.

About the grease stains ..... always clean the surface before you paint. NEVER use white spirit like you mentioned Peter, it is not good for paint prep because it will leave a thin grease film itself. Same with normal soap. There are various special de-greasing agents available, but they tend to be expensive. I prefer to use an ammonia in water solution. It's dirt cheap and it works BEST of all. It leaves a surface that is so clean, it almost feels sticky. Paint will hold very good.

Pen and pencil marks are best removed by sanding or with white spirit or thinner. If you use white spirit, you have to clean after it with ammonia.

 

[joiner1970]

I always use two pack filler less likely to crack and as Jonny says v groove the joint easier to do before but you can do after with a router if needs be.

Another note DONT use ballpoint pens for marking out thats a big no no always use a pencil or knife. The ink will always come through the paint same as when you see a tiled wall and the tiler has marked the tiles with a felt tip marker within a few days the ink will leach into the grout and look awful.

 

[Peter Hamilton-Scott]

...says v groove the joint easier to do before...

Thanks for the advice all. I'm wondering if I should attempt to cut the groove in situ by using a steel ruler as a straight edge and then gradually cut away the "v" using a sharp knife? I've got four joins to do - I wish I'd asked these questions earlier. Doh!

I've also used some dominos and glue here and there so cutting might be my only option then. I'll certainly bear the excellent and most wise clean-up and filler suggestions in mind.

johnny round boy: I looked at your link to screwfix.com but 550g is way too much for the miniscule amount I need. I'll have to look for something else that maybe comes in a small tube. I like your avatar of Homer Simpson. It reminds me of me. ;D

I'm also curious to know if manufacturers intentionally polish/smooth the surfaces of MDF? A few weeks ago I was cutting some scrap MDF to practice my shiny new Domino on. I tossed one offcut on the ground and it landed on another piece. I thought nothing of it until I walked away. As luck would have it I stood on the top piece and I swear the two bits of MDF were slippier than wet ice on wet ice. I landed on the ground about 0.001s later. Bloody hurt it did. For those kind and compassionate enough to ask, the Domino was not damaged. Which also raises another quick question: should I lightly key the surface of MDF with a fine grit paper before applying primer, undercoat and topcoat?

 

[AlexThePalex]

Which also raises another quick question: should I lightly key the surface of MDF with a fine grit paper before applying primer, undercoat and topcoat?

Yes.

 

[Aegwyn11]

You can use just about any filler that is not water-based on MDF.  I generally use Bondo, but have used vinyl spackle, wood filler, etc with success. 

For cleaning the surface, I would wipe it down with denatured alcohol. 

Make sure you use a good oil-based primer, which should cover your pen marks.  I actually like Kilz primer for MDF.  I will put on a heavy coat, sand thoroughly.  Then I will apply a thinner second coat, sand lightly and then apply paint. 

Depending on project, you may want to seal the edges with shellac.  They will soak up a lot of primer unless sealed.

I'm working on painting the MDF pieces of my new ClearVue cyclone. I researched how to clean up the edges online and someone suggested sanding the edges up to 600 grit and using something like Kilz...I tried this on one piece and it seemed to work fine....no stubble raised up on the edges. When I went to sand with 220 grit Brilliant2, it scummed REALLY fast. This was after letting the Kilz dry for about 12 hours, so I figured I just didn't wait long enough. Hopped on Kilz's website and they say the product is not fully dry and should not be abraded until 14 days. How long do you wait for a coat to dry before sanding?

 

[mwhafner]

I'm working on painting the MDF pieces of my new ClearVue cyclone. I researched how to clean up the edges online and someone suggested sanding the edges up to 600 grit and using something like Kilz...I tried this on one piece and it seemed to work fine....no stubble raised up on the edges. When I went to sand with 220 grit Brilliant2, it scummed REALLY fast. This was after letting the Kilz dry for about 12 hours, so I figured I just didn't wait long enough. Hopped on Kilz's website and they say the product is not fully dry and should not be abraded until 14 days. How long do you wait for a coat to dry before sanding?

What Kilz product are you using?  I use Original all the time, and seldom have to wait more than a few hours, even with a thick coat. 

 

[Aegwyn11]

What Kilz product are you using?  I use Original all the time, and seldom have to wait more than a few hours, even with a thick coat. 

Kilz original. Its dry to touch after maybe 30-60 minutes, max. A little more detail, I primed, let dry overnight (about 10-12 hours), and tried to sand smooth the next morning. Using my RO150FEQ in RO mode with speed set to about 3, CT set just above turtle, and brand new 220grit Brilliant2 paper, I got small corms (maybe 1/8-1/4 inch diameter) forming on the paper within 10-20 seconds of starting to sand. I could pop these corms off with my fingernail. Also, they were perfect little circles. That was Saturday...maybe I'll give it another shot tonight and see if they still form.

 

[AlexThePalex]

The scumming will always happen when sanding filler smooth. It's just the nature of the stuff. It will take a long time before it's totally dried out. And with a lot of fillers it will never dry rock solid, but will remain a bit flexible to make up for movement in the wood. It's perfectly normal that your paper scums and there's nothing you can do about it. 

 

[Aegwyn11]

The scumming will always happen when sanding filler smooth. It's just the nature of the stuff. It will take a long time before it's totally dried out. And with a lot of fillers it will never dry rock solid, but will remain a bit flexible to make up for movement in the wood. It's perfectly normal that your paper scums and there's nothing you can do about it.  

I get that, but at the same time it doesn't make sense to me. I've seen lots of remarks that people sand the primer between coats, but with the scumming problem I had that would be totally impossible without using up a box of paper each time. Once the corms started to form (within a few seconds), they would be the only thing making contact with the surface and end up just polishing the surface instead of sanding it. The corms that I had forming with this weren't like normal corms that I've dealt with before where you could keep sanding with that pad for at least a while...once these buggers formed the pad was USELESS...from what I could tell it looked like the corms quickly grew thicker than the grit so you end up with the situation where the corms are the only thing rubbing the surface (instead of the grit).

I just thought of something...could my problem be because I'm using Brilliant2? Should I be using Titan2 as the Kilz is oil based?

 

[Pete Pedisich]

Best results - Zissner BIN Shellac based primer - spray can works best.

Dries fast - soaks in - hardens mdf fibers - topcoat with anything.

 

[AlexThePalex]

I get that, but at the same time it doesn't make sense to me. I've seen lots of remarks that people sand the primer between coats, but with the scumming problem I had that would be totally impossible without using up a box of paper each time.

I know close to nothing about the products you use in America, but after reading what the manufacturer says about Kilz original, it seems to me there is something not right with the Kilz original you used. According to the manufacturer you should be able to sand and paint it after an hour. If the formula generally dries that quick, there is no way it is normal that it will scum your sandpaper after 12 hours drying. Perhaps your can of Kilz was past its shelf life or it was produced faulty.

I just thought of something...could my problem be because I'm using Brilliant2? Should I be using Titan2 as the Kilz is oil based?

There is nothing wrong with Brilliant2. It's the best sanding paper Festool offers for paint, especially oil based paint. I work almost exclusively with oil based paint and do 80% to 90% of my sanding with Brilliant2.
 
For me, it would be very strange to use a product like Kilz. For wood, I never use products that take such a short time to dry. Seems almost unnatural. I use top notch products from the Sikkens brand, made by Akzo Nobel, and the primer takes just as long to dry as the finish paint, a minimum of 16 hours.

 

[peter halle]

Alex,

To explain, and to add to your very wise statements, I offer:

Kilz has several primers out there.  They have a pigmented shellac based primer, an oil based primer, and a latex based primer.  Until we know which product was used, neither you nor anyone else will be able to offer better advice.  A super quick dry would be the shellac.  Probably wouldn't gum up.  An oil based might gum up if it wasn't given proper time to cure in the ambient conditions, and then there is the latex that will probably gum up.

Alex, I love your advice - keep going.

Peter

 

[mastercabman]

The scumming will always happen when sanding filler smooth. It's just the nature of the stuff. It will take a long time before it's totally dried out. And with a lot of fillers it will never dry rock solid, but will remain a bit flexible to make up for movement in the wood. It's perfectly normal that your paper scums and there's nothing you can do about it.  

I get that, but at the same time it doesn't make sense to me. I've seen lots of remarks that people sand the primer between coats, but with the scumming problem I had that would be totally impossible without using up a box of paper each time. Once the corms started to form (within a few seconds), they would be the only thing making contact with the surface and end up just polishing the surface instead of sanding it. The corms that I had forming with this weren't like normal corms that I've dealt with before where you could keep sanding with that pad for at least a while...once these buggers formed the pad was USELESS...from what I could tell it looked like the corms quickly grew thicker than the grit so you end up with the situation where the corms are the only thing rubbing the surface (instead of the grit).

I just thought of something...could my problem be because I'm using Brilliant2? Should I be using Titan2 as the Kilz is oil based?

No, but it is possible that you are sanding too much and too hard.Just sand lightly.If a project is small i sand by hand.You just want to smooth the surface.After the second coat it gets better!

 

[AlexThePalex]

Until we know which product was used, neither you nor anyone else will be able to offer better advice. 

Peter, maybe you missed it, but Aegwyn stated 5 posts above it was Kilz original.

Alex, I love your advice - keep going.

Thank you, nice to hear my input is appreciated. Will do so.

 

[peter halle]

Alex,

Again you are correct.  I missed it.  OOps.

Peter