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Old 14th September 2009, 07:03 PM   #1
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Default Primer For MDF

Hey Guys I am just finishing a Tritrix build and and I am down to the final sanding and painting stages. I have found some conflicting information on proper primers for MDF. Somewhere I came across someone stating that they had very good luck using a Sherwin Williams primer that was able to be built up well and sanded.

However, i can not locate the product name anywhere. I went to my local store and was sold A-100 oil based primer. Is this what I need or do I need to take it back and start with a different product.

Sorry for a question on my initial post...
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Old 15th September 2009, 12:49 AM   #2
sardonx is offline sardonx  Canada
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Hey Vegas. I'm currently building a pair of speakers out of baltic birch ply and and going to paint them. I have put several coats of primer on them. The primer I'm using is Zinsser 123 Bullseye primer/sealer. It is the original water based solution but there is also an oil based one. I'm not sure what the main practical difference is asides from fumes maybe. It has been recommended to me as the best one in home depot (how ever much value you put on that I haven't used other primers but the dried and sanded surface feels very nice. However it has quite a thick consistency and i'd recommend if anyone uses it to thin it according to the directions and apply thin thin coats(which i did not until the last few coats, when it was redundant). I ended us sanding mine going up to 600 grit and it feels very smooth to the touch but looking at it against the light i can see some streaks from the brush still. It's hard to say whether they will show up or not. Bottom line if you prep the mdf really well (filler,sand till you turn blue) and take care with application i think this is a good product. And the mdf should be easier to work with than the ply. Good luck!
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Old 15th September 2009, 01:53 AM   #3
infinia is offline infinia  United States
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Default MDF primer

Yes solvent based primers are what's recommended because of the obvious risks with water and MDF. Sherwin Williams or Zinsser is a good top name brand. Most except the varnish based ones will require initial thinning with mineral spirits.
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Old 15th September 2009, 02:02 AM   #4
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I have had the best results when I used solvent based polyurethane as the sealer, then a solvent based primer. I let the poly dry overnight and sand lightly with fine sandpaper before priming.
This method hasn't let me down.
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File Type: jpg IM001654.JPG (95.8 KB, 462 views)
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Old 15th September 2009, 04:26 AM   #5
infinia is offline infinia  United States
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Using oil based polyurethane as a sealer you could pretty much use any primer on top of that no problem, but at 3 times the cost /gal of an oil based primer, I'd personally wait to use the good stuff as a hard protective final overcoat. I just stock up with the Zinsser B-I-N primer and use that in my gun on everything not needing a lot of sanding/smoothing. Sanding between operations is standard practice, so the goal is to cut down on unneeded steps, YMMV. I can see the point of trying to seal the inside of cabs in high humidity, but it's (poly) not really gonna work for moisture over time anyway ie definitely can't make MDF exterior grade.
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Old 15th September 2009, 04:39 AM   #6
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Gorgeous John!!!

Thanks guys for all this info... helps me too

Cheers!
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Old 15th September 2009, 06:32 AM   #7
infinia is offline infinia  United States
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Yes John looks really yummy. Is that the same finish (tinted and poured on?) as the other thing you did.. can't remember if it was an amp or computer case? I think we need a permanent link for that finish and the bending MDF stuff too. Thanks
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Old 15th September 2009, 02:29 PM   #8
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Quote:
Originally Posted by MJL21193 View Post
I have had the best results when I used solvent based polyurethane as the sealer, then a solvent based primer. I let the poly dry overnight and sand lightly with fine sandpaper before priming.
This method hasn't let me down.
Damn that looks good!

Care to post the steps required to achieve that type of piano finish? I don't expect my speakers to look this good but I will like to achieve some type of high gloss finish.

So far I have:

1. Filled all of the holes and joinery lines with wood fill. Sanded with 80 GR. and refilled.

2. Applied a 50/50 water/glue mixture to the cut edges and surface of the MDF.

3. Tonight I wanted to do a final surface sand maybe with 100 GR and then 220 and prime.
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Old 15th September 2009, 11:51 PM   #9
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Quote:
Originally Posted by loudvegas View Post
Damn that looks good!
So far I have:

1. Filled all of the holes and joinery lines with wood fill. Sanded with 80 GR. and refilled.

2. Applied a 50/50 water/glue mixture to the cut edges and surface of the MDF.

3. Tonight I wanted to do a final surface sand maybe with 100 GR and then 220 and prime.
Thanks,
I have gone thru this on other threads but here goes again:
First, MDF doesn't like water. Water makes it swell so your 50/50 PVA/water mix is a no no. You should try to keep the MDF as dry as possible and keep it in a dry place so as it will not pick up too much moisture from the humidity in the air.
I use solvent based (not waterbased) clear polyurethane to seal every MDF surface, inside the enclosure too. This can be sprayed or brushed on. Why polyurethane? It doesn't separate when it's absorbed into the wood grain. Other, oil based coating may separate (the solvent - oil - gets absorbed into the wood grain and what's left sits on the surface). Polyurethane is VERY moisture resistant - it forms a barrier and keeps the MDF at a constant moisture percentage even when the humidity in the air around it goes up and down. It's a very important first step for a great paint job on MDF.

Primer. Once again I like to use solvent based primer. It takes longer to cure but is an added barrier against moisture (water is the enemy). I spray 10 or more coats, letting each coat flash (dry/tacky to the touch) before spraying again. This can take a few hours, depends on how warm the temp is.
After the case is primed, let it cure for several days. It should be ready to sand smooth when it resist a fingernail dent and the sandpaper doesn't get clogged up when sanding it. I dry sand the primer to 320 grit.

Topcoat. The speaker in the pic above was sprayed with 10 (or more) coats of water based black polyurethane. This is a special order from Sherwin Williams called "Armorseal". It is high gloss industrial floor paint. It is tricky to spray because it is so thick (reducing it too much weakens it) but it dries VERY fast and polishes (as seen in the pic) very well.
Another option that I have explored is a solvent based colour coat (3-4 coats) and a solvent based clear over that (6-10 coats). The clear can be clear polyurethane. As usual, give it several days before colour sanding and polishing.

Hope this helps.
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