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Sanding MDF for paint

Louis Brandt

We use MDF for painted slab doors and cabinets. Our finisher insists on sanding the faces of parts to 320g prior to finishing. The edges are banded with maple veneer. We are spraying opaque catalyzed conversion varnish from ML Campbell. My hunch is that the parts only need to be sanded to 150 prior to primer. We're doing 120g, 150g, 180g, 220g, 320g. All with a Festool orbital. Seems excessive to me. What do you think?

11/17/15       #2: Sanding MDF for paint ...

sanding that high will cause adhesion issues. your instincts are correct. we sand to 150 before primer to allow the primer to bite real good.
not only are you wasting time and material, you are actually creating a surface that is to polished to accept a coat.

11/17/15       #3: Sanding MDF for paint ...
Louis Brandt

Well, that explains why my finisher doesn't like blue tape; he says it's too sticky an pulls the finish off.

11/17/15       #4: Sanding MDF for paint ...

Yup, Kevin is right, you can find the specs for ML products online, print them out and hand them to your finisher. IIRC 150 is about the highest you should sand to, I usually do 120 grit, 2 coats of Clawlock, then 2 coats of Resistant for what I consider a very nice finish.

I wouldn't use blue tape either until it's dried at least a couple days. It shouldn't lift the finish but it will leave a discoloration on the surface covered if used to soon….I learned that one the hard way!

good luck,

11/17/15       #5: Sanding MDF for paint ...
Adam West  Member


320 on the compressed face of MDF is not necessary at all. I do often use 320 for the edges and routed profiles and it works wonders for smoothing the texture so the MDF won't suck in so much finish.

150 grit on the face is too rough. Too coarse a grit will pull up the fibers that you want to leave in place in the tight surface. 220 is a better choice. You don't want to grind off much of any material. Just remove the scratch pattern from the mill that made it and get rid of handling marks.

11/18/15       #6: Sanding MDF for paint ...
Louis Brandt

I just called tech support at ML Campbell, and they recommend 320g for the faces of MDF, 400g for the edges. Not what I wanted to hear.

11/18/15       #7: Sanding MDF for paint ...
Rick Mosher

Do your own adhesion test and find out for yourself what works and what doesn't.

Adhesion testing

11/18/15       #8: Sanding MDF for paint ...
Joe Member

My ML Campbell finishing rep had recommended this some time back. I use the Agualente waterborne. Sanding at the 320 and 400 end grain, has never failed me.

11/18/15       #9: Sanding MDF for paint ...
Rick Mosher  Member


A WB product would be different than a solvent based one. The water will raise the grain a little promoting adhesion better.

11/18/15       #10: Sanding MDF for paint ...

i have to admit i am absolutely shocked. You mean to tell me that ive been sanding ALLL my mdf wrong this whole time? I mean, i couldnt even begin to count how many miles of mdf panels i run thru my heeseman sander with a 150 grit belt every day right before they hit my cefla to be primed, then back to the heeseman for scuff then back to cefla for paint.

Not sure what works for you, but i like conversion varnish that adheres so well it takes a jack hammer to chip it. And i get that from 150.

If you take the advice of a professional finisher, you get good finishes. Take the advice of a sanding supply salesperson, you buy more sandpaper.

11/18/15       #11: Sanding MDF for paint ...


i just pulled this off the ML Campbell website. This is in the data sheet for clawlock primer :

Surface Preparation
New Work:
Remove any dirt, grease, glue or other construction contaminants and sand wood as
required. When using high solids coatings on wood, it is important that the wood retains a
profile after sanding for the coating to lock onto. High solids coating like Clawlock require a
slight profile left on the wood after sanding. For best adhesion, finish sanding on wood,
should be done using a maximum of 150 grit paper. Always be sure that sanding belts and
sand paper used are not worn, as worn sanding materials tend to polish the wood.

---- " For best adhesion, finish sanding on wood,
should be done using a maximum of 150 grit paper"

11/18/15       #12: Sanding MDF for paint ...

heres the data page

ML Campbell Clawlock Primer

11/18/15       #13: Sanding MDF for paint ...
Louis Brandt

Yeah, I saw that on the website. They also had an additional section about MDF:

Remove any dirt, grease, glue or other construction contaminants and sand wood as
required. Routed areas on MDF board should be sanded with a fine grade of sand paper,
a minimum of 400 grit. UV filled MDF board must be sanded before application of
Clawlock Primer / Undercoater to assure good intercoat adhesion.

This is why I called them. They mention 400g for edges, but don't mention anything specific for the faces, so I was unclear on if the recommendations for "wood" applied to the faces of MDF. They guy in tech support looked it up and told me 320g.

In any case, I'm going to do some tests with various levels of surface prep and see what kind of results I get.

11/18/15       #14: Sanding MDF for paint ...
Paul Snyder  Member


I usually use 150 for pretty much everything, but like Adam I also prefer 220 on MDF to avoid making the surface too fuzzy. I also use the same grit on the edges so it won't soak up all the primer. This single sanding step works for me and the adhesion is good - no problems.

The picture shows an example where I sanded the right half of the panel edge with 220 and the primer didn't soak in like it did on the left side (sanded to a higher grit).

View higher quality, full size image (681 X 511)

11/18/15       #15: Sanding MDF for paint ...

So what your saying is the "tech" you spoke to contradicted ML's printed recommendations? I'm not sure I'd want to talk with that guy again!

All I can tell you is I've got a lot of mdf panels out there and not a single one has ever been sanded past 150, and most just to 120 grit. I've never had a problem….but that's just my experience. Oh and edges/profiles are completely different, then you do have to up the anti in terms of grit or it's a real pain to get a smooth finish.

good luck,

11/18/15       #16: Sanding MDF for paint ...
Adam West  Member


I am one of those sand paper suppliers that must only want to get you to buy more sandpaper according to Kevin.

My reputation in the industry is one of saving customers money.

Sanding to 120 or 150 grit can work. It just uses more material to fill the surface. You can sand with 80 grit if it tickles your fancy.

MDF likes to be polished a bit to help avoid too much material soaking in. The more you fluff up the surface the more the material soaks in.

I work with ML Campbell, Chemcraft, Akso Nobel, Sherwin Williams, Gemini, and several other coatings companies. They all have their various ways of trying to fill the grain on MDF.

Too much solids and the material won't flow out and you will have bad orange peel. Too thin and it just doesn't fill.

The best MDF flat surface I have seen so far was sanded to 220, sprayed with a light coat of thin primer to seal the surface, sanded to 280 grit, then primed again with a primer with good flow and the surface will look much flatter and cleaner. 280 again before paint.

The best profiles I have seen were sanded to 320 first. Then sanded with a 150 grit double sided sponge after the first and second coats. The equivalent of 320 grit again.

I hate to watch people load on huge amounts of primer, leaving a nasty lumpy mess, and then sand most of it off in an attempt to get the surface flat again. I watch customer after customer waste 80% or more of their primer by sending up the dust collector and they never get how insanly silly that is......

11/19/15       #17: Sanding MDF for paint ...
Louis Brandt

I spoke with ML Campbell again this morning, and 320g. is the recommendation. See the attached MDF Guide. After seeing all of your posts, I started to wonder whether what the tech I spoke with was referring to the grit to use when sanding between coats when he told me 320g.

Click the link below to download the file included with this post.


11/19/15       #18: Sanding MDF for paint ...
Rick Mosher  Member


On the face of MDF you are sanding between coats of finish as there is a UV coating already on the sheet.

11/19/15       #19: Sanding MDF for paint ...
Louis Brandt

We are sanding between coats of primer and finish, 320g I believe. My question was how much the face of the MDF needs to be sanded before the first coat of primer is applied. My finisher and ML Campbell both say 320g. I'm not sure if the MDF we use is UV filled or not.

11/19/15       #20: Sanding MDF for paint ...
Paul Snyder  Member


Louis - the data sheet they sent you does say to sand the face of raw MDF with 320 or to use 180 if it's pre-primed from the factory (UV filled). You would know if you're using pre-primed/sealed MDF. The sheet goes on to say that you should pre-seal the MDF with a coat of highly thinned Magnalac if it's a lower density product that gets very fuzzy when sanded. That's an extra step and I don’t know anybody that does it. Let me point out that the data sheet does not say to work your way through all the grits up to 320. It says to use 320 (only). That's a very important distinction that will save a LOT of time and material immediately.

It may be that the technique outlined in the data sheet is the best approach. It's also possible there's a better way. In this thread some experienced finishers offered their advice and they use different approaches to achieve the same results. That tells me there’s more than one to get the job done. The one thing most everyone seems to agree on is that 320 is too fine.

Chances are some of the approaches are more cost-effective than others in terms of efficiency and/or cost of materials. Though sometimes a savings in material is offset by an expense in labor (and vice versa). It's a balancing act.

At this point I’d recommend you do some testing to find the approach that is most cost effective and produces the standard of quality you need to make the customer happy. There are some variables that can affect what will work best for you including the grade of MDF you use, the techniques used by the people doing the sanding, the type of the sandpaper, the techniques the spray operator uses, etc..

Start by sanding half of a 4x8 panel with 150 and the other half with 220. Do not work your way through all the grits - use these single grits ONLY. Then sand the edges on one half with 220 and use 320 on the other half. Perform the work quick and easy – do not take extra time to make it better than usual.

Now spray the whole panel with a coat of primer thinned about 20% (or more if that’s your standard procedure). Sand the panel with 220 or 320 to smooth and spray another coat of primer (thinned per your preference). Sand with 320 to smooth and topcoat.

Let the panel dry for 3 days in a warm, well ventilated space and then evaluate the visual difference between the 2 halves. Do an adhesion test on each half. Choose the side that looks the best, adheres well, and takes the least amount of work/time. Start using that process on a regular basis but keep an eye on the quality and labor hours. Do another adhesion test in couple weeks just to follow up. As a matter of routine, make sure the sanding crew isn’t using worn out sandpaper.

Find out if your company always orders the same quality MDF - some are denser than others and are easier to paint (less coats of primer). The standardized process you come up with will have to adapt if your supplier sends you a different grade of MDF one day, so you should be familiar with what you’re getting and how it behaves from a machining and finishing perspective.

Lastly, if you get a chance, report back on the results of your testing and the process that works best for you.

11/20/15       #21: Sanding MDF for paint ...

i like the above post. see what works for you.. .

and on a side note, i would be weary of any process that produces finishes that cannot hold up to blue (low-tack) tape.. if the paint is pulling off by using blue paint you have an adhesion issue, and the process must be reviewed.

11/20/15       #22: Sanding MDF for paint ...
Louis Brandt

Guys, thanks for your help, this has been great. I'm going to take this to the lab and do some serious testing. I'll post my results. Thanks again.

11/22/15       #23: Sanding MDF for paint ...

I use Becker Aroma and my supplier for years has said don't sand anything past 180.
It doesn't matter what grit you sand with, you are going to get grain lift, that is what liquid on wood does. Thats when you give it a good sanding with a fine grit, I use 320 for clear and coarser for solid color.
In your case the quality of finish sounds like all your concerned about so as others have said, just finish a couple panels that have been sanded to 150 and 320, Complete your schedule for finishing and compare them. You then have to decide if the extra sanding is important for your finished product or is it an extra expense? good luck.

11/24/15       #24: Sanding MDF for paint ...
Denny J

Claw lock would certainly require less prep sanding than Aqualente. Personally I have shot tons of both and have found that 150 grit works for us and gives great results. I find that a thinner first coat makes a bigger difference than using that many sanding levels.JMHO

12/4/15       #25: Sanding MDF for paint ...

We've always done 150 grit for wood.

If we are trying to get a good quality finish on mdf like raised panels. Then we sand the exposed edge to 240 grit. Spot prime with BIN or Clawlock. Sand with 320 then spray the whole panel.

There should be no adhesion issues on exposed edges of mdf. Its like a sponge. The finish is literally absorbed.

12/4/15       #26: Sanding MDF for paint ...

We don't sand the original faces ever. The material is so porous the idea of roughing it up to promote adhesion is not relevant.

12/4/15       #27: Sanding MDF for paint ...
Louis Brandt

My test samples are in the finishing room now, I should know soon what works best.

12/31/15       #28: Sanding MDF for paint ...
Steve G.

It is time to move to the 21st century,as a finisher for the ultra high end furn.industry for over thirty years the best method to finish mdf. is to sand the edges to 320 and the surface to 320 as well,and seal the MDF. first , with either a Catalyzed vinyl sealer. S.W makes the best, reduced to penetrate into the MDF.or use Ilva poly sealer.
Sand to 320 then prime. When you seal first there will be no micro voids bet the primer and the MDF.therefore no more shrinkage and your edges will look totally filled forever.And by the way the primer really holds on to the sealed MDF.NO masking issues.

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