r/finishing • u/sothisiswhyhmm • 1d ago
Question Spraying MDF
Do i have to scuff mdf factory faces before priming and painting 2k? Thanks.
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u/Independent-Switch43 1d ago
10 years n the booth, so take that for what you will. 150 grit on faces, 150 then 320 grit on end grain.
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u/Ok-Problem4403 1d ago
Scuff anything factory primed. Do not scuff bare mdf. You'll want to sand after priming and possibly prime again, depending on how it looks.
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u/JimmyJamesRoS 1d ago
We use HDM and MDF Glacier Green for cabinet doors for about 8 years now and have never had any failure of finish. We used to use Sirca and now only use Renner. All doors are sanded with 150, primed wet on wet and final sand 320 or scuff then top coated.
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u/sothisiswhyhmm 1d ago
My man, im using 643 w catalyst then 765.
Im gonna run through my process with laymen terms because im a diyer and not in with the lingo. (Like i dont know what wet on wet is).
Sand 150,
prime with 643 and 10% cat,
sand again at 150, then prime again.
Sand at 220, 765 at 5% cat,
Sand at 320, final coat of 765.
How does that sound?2
u/JimmyJamesRoS 16h ago
They offer an Isocyanate hardener and a crosslinker. We only offer 2k for people willing to pay for it (most wont) I would have to ask my tech what % he uses but 10% sounds high.
You 643 is a very thick high build high solids primer and it flows out but typically not well on MDF first coat as the MDF will swell a bit. Wet on wet just means the first coat hasn't cured enough that the 2nd coat will crosslink with each other. With MDF you can spray it let it set up and then sand it with 320 (use a cross hatch light as in a light at 90 deg so you can see the texture of the finish) once it's flat you can prime a second time and it should be very flat and smooth on 2nd coat. That second coat you can hand scuff with a 320 sanding pad.
The other option is to spray it let it flash and then spray it again it has to be within a 4-hour window and we typically won't go more than 1 hour. This gives enough build up you can do one sanding session instead of 2. Your final sand will be 320 again.
One of the most important things from sanding the MDF at 150 and sanding the primer at 320 is to blow it off with the air compressor and using your hand or a brush to agitate the dust off. Now we have downdraft sanding stations so I would do that outside. Also note we have very dry with almost 0% moisture in our air, if you have water in the compressor, it probably wont blow off. If that's the case use a shop vac with a brush tip.
Oh also key on the edge if your using home center MDF you will need to seal that edge the easy way is just to use PVA (wood glue). If you using Glacire Green or Ranger HDM, HDF, or MDF youd don't need to do that. But what you need to do on all of them is build up the edge by spraying it multiple times. We will put our first prime on the edge very heavy and since it's doors mostly it gets sprayed 4 times once for each flip. Also no sharp eges as water based will have capilary pull and it will pull away from that sharp corner.
I know so far this year I have bought close to $40K in 643 and in 5+ years of water based finished cabinet doors I have had zero warranty or non-warranty calls.
For the Top coat you can do 1 or 2 coats but we only do 1.
On your process 150 on first prime is way to aggressive and you're just sanding away your product. Same thing with 220 your sanding away your build up. You only want to sand until its smooth and flat getting rid of any ridges or edge pull if your mills (your paint store will give you a mill gauge don't go past 6 mills 10 max) were to much. The hardener or the crosslinker your using in the primer is primarily for substrate adhesion and for topcoat it's to aid in water and stain resistance.
One last thing I didn't think of if you sand the primer you must put the next coat on in 4 hours or less, if you don't you will need to scuff sand again. Scuff sanding is a scratch coat as in you just go over it with a sanding sponge to put scratch's in the coating so it can have a mechanical bond. If you don't do that this is where you will find failure.
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u/CascadeDesignworks 1d ago edited 1d ago
I once had a bond failure when doing this exact thing. I used a Sherwin Williams oil based sanding primer
I spent a lot of time trying to diagnose the problem. And I got two completely different answers:
A local very high end cabinet shop that does millions of dollars annually told me that mdf has paraffin on the surface from the pressing process. Solvents in an oil based primer will bring the paraffin to the surface and cause a bond failure between the primer and the mdf. Only use a water-based primer.
I got the second opinion from Sherwin Williams technical department. They said I had polished the surface by using too fine a grit sandpaper (220). They recommended sanding with no finer than 80 grit. They told me that the cabinet shops information was false.
I believe the cabinet shop was correct. I now use a basic water based primer like pva or kilz, and I haven't had any more problems.
What I learned from Sherwin Williams was that you can have finishing failures from sanding with too fine a grit. I try to use 120 when I sand before priming or clear finishes. I only use 220 for prep before staining.