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Painting MDF

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Forum topic by plang posted 12-25-2017 11:14 PM 789 views 0 times favorited 13 replies Add to Favorites Watch
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plang

117 posts in 3411 days


12-25-2017 11:14 PM

Just want to know how to paint MDF. I recently purchased a sheet to make shelves for my grandsons hotwheels. In the process of spraying them the mdf felt gritty after painting even with primer. What am I doing wrong. Thanks


13 replies so far

View EugdOT's profile

EugdOT

226 posts in 611 days


#1 posted 12-25-2017 11:27 PM

Mdf is very very dusty. Might be some residue fine dust that never was cleaned off? You could hit it lightly with 120 or 220 sandpapered to see if you can get it smoother

View Rich's profile

Rich

3206 posts in 646 days


#2 posted 12-25-2017 11:42 PM

You need to provide more details about paint type, how you sprayed it, etc.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View CaptainKlutz's profile

CaptainKlutz

323 posts in 1550 days


#3 posted 12-26-2017 05:35 AM

Been there done that – Painting MDF is more work than it seems.
Why? MDF acts like a sponge when you try to paint it.
Hope you did not try to use water based paint?

IMHO – You have 2 choices painting MDF for a smooth finish;
use 3-5 coats of paint, sanding between coats, or
use primer to seal and prep the surface, then apply 1-2 coats of top coat.

You can achieve a full gloss (polished) finish on MDF spraying oil based top coat (or enamel rattle cans) when primed first. Have made many white laminate garage cabinets with MDF doors using spray cans.

Choice of priming paint depends on top coat. Shellac works best on MDF (IMHO) for most any paint, oil or water based. If painting object white or can tolerate a white base coat, Zinsser makes a white shellac based primer that can be brushed on to seal MDF. I would not recommend any water based primer paint as it creates fuzz that makes for more layers of sanding/painting.
After shellac priming, 150/220 sand surface as prep for top coat. If edges are rough, can use wall board compound to fill them, then sand and prime to seal the sanded filler surface. You will end up with a perfect surface to top coat that will not create fuzzy surface even with water based top coat.

Best Luck.

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

View jar944's profile

jar944

127 posts in 1494 days


#4 posted 12-26-2017 03:24 PM

I can’t say I’ve ever had any issues with mdf (except for the cut/shaped edges)

Zinsser BIN would be my primer suggestion, though I’ve had issues with it lately being gritty out of the gun and or clogging strainers.

Lenmar duralaq nitrocellulose undercoater would be my personal choice, but it’s solvent based.

View bilyo's profile

bilyo

247 posts in 1159 days


#5 posted 12-26-2017 09:49 PM



Just want to know how to paint MDF. I recently purchased a sheet to make shelves for my grandsons hotwheels. In the process of spraying them the mdf felt gritty after painting even with primer. What am I doing wrong. Thanks

- plang


The grittiness Is likely due to raised grain which will happen regardless of primer use; more so with water based. With most any primer, do a light sanding with 220 grit or finer before the finish coat. If your finish coat is water based, wipe the dust off with a water dampened cloth or paper towel. If the finish coat will be an oil base, dampen with mineral spirits.
I second the use of Zinsser BIN. I works with water or oil based finish coats and sands easily to a smooth surface. I do very little spraying, but have had better results applying the BIN with a foam roller rather than with a brush only because I get a more even coat.
As CaptainKlutz mentioned above, your cut edges will likely be rough and porous. You can fill these with wallboard joint compound or spackle and sand smooth before priming. With either one, be careful wiping the sanding dust off; both are water soluble and will come off if your wiping cloth is too wet. They will be fine once primed.

View MrUnix's profile

MrUnix

6830 posts in 2255 days


#6 posted 12-26-2017 10:12 PM

The grittiness Is likely due to raised grain which will happen regardless of primer use; more so with water based.

Wasn’t aware that MDF had ‘grain’ :)

I’ve never had any problem painting MDF or any issues with routed edges, but I have never used anything other than oil based enamel. Spray primer will leave a slightly rough/dusty feeling surface – it’s just the nature of the beast, and it needs to be knocked down before applying a finish coat. I just spray multiple coats of the finish paint as I’ve never felt that primer does anything that regular paint wouldn’t do anyway, and don’t see a need to purchase two products when one works just as well. Don’t have many pictures, but here is a bandsaw base made with MDF and IIRC three coats of enamel… as you can see, it’s almost a mirror finish:

If you want to see some truly impressive stuff made from MDF, check out this thread over at OWWM, where most of those faux bases are made from MDF:

shopmade machine bases: the cream of the crop

Not exactly shelves, but still darn impressive, and shows the wide range of finishes you can get with the stuff.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View bilyo's profile

bilyo

247 posts in 1159 days


#7 posted 12-26-2017 10:57 PM


Wasn t aware that MDF had grain :)

Brad

- MrUnix


Is MDF made from wood? Does wood have grain? ;>)

BTW, I meant to mention that if you use BIN, which is white, you can sometimes get your supplier to add some color tint to darken it. This can be preferred if your top coat is a dark color.

I have also been known to use a top coat as a primer. However, I’ve found that some are difficult to sand; especially latex or acrylic. In my experience, most primers sand much more easily.

View Rich's profile

Rich

3206 posts in 646 days


#8 posted 12-26-2017 11:58 PM


Is MDF made from wood? Does wood have grain? ;>)

- bilyo

You’re kidding us, right?

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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bilyo

247 posts in 1159 days


#9 posted 12-27-2017 06:39 PM

You re kidding us, right?

- Rich

Partly. But, MDF is a wood product with wood fibers that can be affected by moisture. Not the same as wood grain per se, but with similar results.

View mayday3374's profile

mayday3374

15 posts in 309 days


#10 posted 01-08-2018 06:06 AM

Recently did a lot of trim work using sheets of mdf ripped to size. The faces Were not an issue but the cut edges were like 80 grit paper after priming. My solution was to use a bonding primer (Valspar from Lowe’s worked for me). 1 coat, ( it will be rough,) sand, and the finish coat went on nice and smooth. I’m guessing any of the other suggestions that were mentioned will work, as long as it seals the cut edges.

View John Smith's profile

John Smith

1135 posts in 219 days


#11 posted 01-08-2018 05:19 PM

all very accurate comments above.

fiberboard should not be confused with particle board.
if you really need to know the differences; google ”fiberboard vs particle board”.
both are manufactured from sawmill byproducts using the same manufacturing process.
fiberboard is wood chips and paper scrap that is ground into almost a powder and glued together.
particleboard is only wood chips with a coarse texture and holds more air space than fiberboard.
both are extremely heavy for their size. both are used in the laminated furniture industry.
both can be activated with water or water based products, causing swelling and raised “grain”.
neither one is approved for high humidity applications without the proper preservation.
a diluted solvent based primer should be used to penetrate the board and allowed to cure thoroughly prior
to application of the desired topcoats instead of a waterborne primer.
fiberboard has excellent working characteristics with routing smooth edges where particle board
is less desirable and requires more fillers to make it smooth. (and thin material does not hold fasteners well).
so if you are experiencing finish problems with either, it may be from the initial priming process.
again – waterborne products will raise the fibers and give a less then desirable finish in both products.

Brad – that link to MDF projects is very interesting !!!

on the subject of moisture-resistant Fiberboard and Particleboard, if you really need to use one
for exterior use or in very high humidity areas, there are products used in the sign business
that meet the demanding exterior requirements. . . .MEDEX and EXTIRA are two of them.
Extira and Medex is medium density fiberboard (MDF) that has been specially treated for exterior sign panels.
waterproof glue is used in the manufacturing process. Found mostly in sign supply warehouses, they are probably a special order item at high end lumber yards. these products are EXTREMELY dense and heavy. impervious to moisture and will hold any kind of primer and paint well. very stable in most environments.

.

.

-- Graduated Valedictorian from the University of HardKnocks --

View John Smith's profile

John Smith

1135 posts in 219 days


#12 posted 01-08-2018 06:39 PM

from: FineWoodworking.com

“Solvent-based primers (oil, alcohol, or lacquer-based) are a must. Never use a water-based product
for the initial finish coat. The wood fibers will swell too much when they absorb the water,
and you’ll get what is, in effect, ”raised grain” on the surface that will not sand out. After the surface
has been sealed with something else, though, a water-based paint will not affect the MDF adversely.”

.

-- Graduated Valedictorian from the University of HardKnocks --

View ArtMann's profile

ArtMann

978 posts in 872 days


#13 posted 01-09-2018 07:02 PM

Actually, Extira is widely used in new house construction and is readily available at builder’s supply places where I live. I am using a small amount of it on my new house.

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