Car body spray on MDF?

[fatmarley]

Hi Guys,

I want to achieve a high gloss finish on my diy MDF speakers. A friend who is a car body repairer said he'd be happy to spray them for me and he obviously has all the gear to do it but he only sprays metal, so doesn't know what paint he should use on MDF.

He wrote down a list of questions:

Is 2 pack paint (also known as 2k or polyurethane car paint) suitable for MDF?

Is 2k or 2 pack primer suitable for MDF? and has it got any flexibility? If not, can a plasticiser additive be put in?

The reason for the flexibility is because I've seen hairline cracks appear on the joints of speakers that some diy guys have sprayed.

 

[mailee]

Yes you can use 2K car paint on MDF with the appropriate primer and sealer but no matter what you do you will still get the cracks appearing on the joints maybe only thin cracks but wood and sheet goods do move. HTH. :wink:

 

[fatmarley]

It's good to know I can use 2k paint.

Any recommendations on a primer and sealer?

There must be a way around the cracking problem because some loudspeaker manufacturers use high gloss finishes. Perhaps they don't use MDF?

 

[Mike.S]

May be worthwhile reviewing the paints on offer at Morrells - both their interior wood and VLi finishes. They're also happy to provide technical advice.

 

[blackrodd]

I built a shaker kitchen a couple of years ago, and found a local painter/sprayer on furniture to do the job, I was pleased to be very busy, at the time.
I know there is a special edge primer and no doubt some paint maybe more suitable than others on mdf .
I just googled mdf paint and 3rd one down, paint doctor has plenty of advice HTH Regards Rodders

 

[fatmarley]

Had a look at the paint doctor website and he said the best way of sealing an MDF edge is to fill it and then sand. I wonder if some wet rot wood hardener would stop the edge from fluffing up?

 

[blackrodd]

Had a look at the paint doctor website and he said the best way of sealing an MDF edge is to fill it and then sand. I wonder if some wet rot wood hardener would stop the edge from fluffing up?

Did I also read that the purpose primers stopped the gasses that mdf gives off?
Rodders

 

[rdesign]

glue and screw the speaker together then use bondo car filler on all screws and joints and edges of mdf as it takes paint differently then sand to 220 then prime this should reduce/ eradicate any joint cracking.

 

[mailee]

I can vouch for Morrells paints and primers they work well. There are two sure fire ways to seal the edges of MDF the first is Morrells MDF sealer and the other is Zinnser bin. Both of these will seal the edges of MDF within one or two coats. Morrells sealer is nitrocellulose based and the Zinnser bin is shellac based both can be over coated with most paints. As has been stated above a car body filler can be used on the joints and this should be a polyester based one to allow for flexibility. There is no guarantee it will not crack later in it's life but this is the best approach to the joints. When i build units in MDF I always use MRMDF (moisture resistant) as this is more dense than the standard 'fluffy' stuff. I sand the edges down with 120 grit and then give them a coat of the sealer. With the MR I can then go on to priming the edges and use Morrells 2K high build primer. I spray one coat on the edges and one on the face, then turn it over and spray a coat on the opposite face and another on the edges. This dries within an hour and then I repeat this. I then sand everything down with 320 -400 grit and apply two top coats using the same method again Morrells 2K. This then gives me a finish ready for flatting and polishing to a high gloss. HTH. :wink:

 

[sploo]

This is hard. Really hard. I've lost count of the number of speaker boxes I've built over the years, and getting a good stable finish on MDF is always a headache.

The major issue is that the end "grain" will soak up loads of paint/primer, and has a tendency to move over time - you'll see the join line with side panel sink.

Things that have helped are:

• Put the finished (but unpainted) boxes in the room where they'll be kept a few weeks before (to try to get the moisture content closer to their long term state)
• After applying several thin paint coats, leave it to cure fully for a few weeks before flatting and polishing (again in the intended environment)
• I've had good success with a 2k urethane paint by Isolack - used both as a primer and paint (basically enough thin coats to soak into end grain, leave to dry, sand, then paint again)
• 1k urethane seems to work OK too, but doesn't go as hard
• Epoxy paint used for model aircraft seems to work well too - but it's expensive
• I've not had much luck in getting a good base for a glossy final coat using Zinsser BIN or Bullseye (even though I love them for other priming jobs)
• A US builder I know has reported good success with (IRC) Varathane floor varnish as a primer, and water based automotive paints (no clearcoat, just polishing the colour coat). Unfortunately that varnish isn't available in the UK
• Some builders use cheap wood veneer to face a panel; such that it covers the ends of the MDF; use a fine automotive filler on the veneer grain, then paint

Just my $0.02

 

[n0legs]

I've used car paint loads of times on MDF and have had no problems with "end grain" sinkage.
Seal, Seal and Seal all the ends, fixing holes, routed patterns etc on the MDF with 2k paint, sand back, prime and paint.

Below is my bathroom cabinet made from MRMDF painted with Lesonal 2k paint in a custom white colour. It's in my bathroom that gets used everyday by the four of us, the bathrooms at a constant 20-ish c air temp and as humid as anyone else's bathroom. No movement, no show through on any of the fixings and no fluffy edges or swelling.




If I remember I'll get a photo of it in situ, it's been there at least a year probably longer.

 

[sploo]

I've used car paint loads of times on MDF and have had no problems with "end grain" sinkage.
Seal, Seal and Seal all the ends, fixing holes, routed patterns etc on the MDF with 2k paint, sand back, prime and paint.

The key there (in my experience) is the 2k paint (and the sealing, of course).

All my attempts at other primers and paint have ultimately failed (with sunken join lines). The problem is that 2k paint is nasty stuff to spray, and as an amateur with no dedicated spray booth I'd rather use something that's a little less like nerve gas when airborne.

Funnily enough, I was doing a bit of tidying of the garage today, and came across a load of MDF blocks I made a few years ago; each one was about 10cm in diameter, and consisted of three layers of 3/4" MDF. The different blocks were glued and sprayed with a variety of products. Only the 2k primed/painted block still looks good. Oh, and the one I covered in West Epoxy before painting (with non-2k paint); but that was a b*llache to do, so I discounted that as a solution.

 

[mailee]

This unit was prepared and painted as I described using 2K Morrells paints and primer/sealer.

 

[fatmarley]

Thank's everyone who's replied. I sent an email to Morrells and they suggested 2 appplications of Profile UHS Sealer/Primer followed by Wet Look 2 K PU or Gloss 2 Pack AC lacguer (I think I'll go for the Wet Look).

They also said that the depth of gloss can be enhanced by using a clear lacquer with approx 5% of the colour as a final coat, although this might not be possible with white or pastel colours.

I've just purchased a random orbital sander and from watching some Youtube videos I think I need to work my way up through the grades of sandpaper until I get to 120 grit, then put a coat of sealer on and sand with 120 again? (I need to check that Youtube video again ) and put another coat of sealer/primer on.

 

[Stoatie]

I'm just about to embark into the wonderful world of spraying and have probably been watching the same youtube videos. I think you're missing a 0 off the 120 grit, should be 1200.

 

[sploo]

I'm just about to embark into the wonderful world of spraying and have probably been watching the same youtube videos. I think you're missing a 0 off the 120 grit, should be 1200.

For polishing a final colour or clear coat yes, but for earlier primer jobs (especially cutting through MDF "fuzz") 120 grit is probably OK. Personally I'd get the primer to 240 grit (at least) before putting on a colour coat - but not too polished, as you want the coat to stick.

 

[n0legs]

I'd get the primer to 240 grit (at least) before putting on a colour coat - but not too polished, as you want the coat to stick.

Aim for an 800 grit finish on primer before a colour coat, the paint will still adhere.

 

[fatmarley]

It looks like there are a number of different ways to achieve a high gloss finish. This is the video I watched - https://www.youtube.com/watch?v=ySPLBjFCf30

He says to give the MDF a light dust coat of primer >Sand with 150 grit to remove all basic? primer (fill any holes if necessary) > spray a coat of catalyzed primer > sand with 320 grit > coat of primer > Leave 24hrs and then sand with 400 grit (mesh or net sandpaper) > Spray with base coat > Sand with 600 grit to remove all of the glossy surface and then put on your final coat.

 

[n0legs]

I should add that my knowledge and experience is from the car body repair world, brother in law has a body shop.
Makes life nice and easy for most of my projects as I get to use the workshop as my own.

 

[sploo]

It looks like there are a number of different ways to achieve a high gloss finish. This is the video I watched - https://www.youtube.com/watch?v=ySPLBjFCf30

He says to give the MDF a light dust coat of primer >Sand with 150 grit to remove all basic? primer (fill any holes if necessary) > spray a coat of catalyzed primer > sand with 320 grit > coat of primer > Leave 24hrs and then sand with 400 grit (mesh or net sandpaper) > Spray with base coat > Sand with 600 grit to remove all of the glossy surface and then put on your final coat.

Kinda feels to me that you'd want much more in the way of primer/sealer to plug up the MDF end "grain" (as it soaks in loads of liquid), but otherwise that doesn't seem too crazy. (EDIT: Looking at the video, the first comment on YouTube is "wish you showed finishing the edges or some sort of details... not to take away from your finish, but on a largish flat surface it's pretty easy to get it close to perfect, but I've always had much bigger difficulties with details and edges (enough so that I usually "cheat" an try to make sure they are all covered up with another material)"; which sums it up pretty well IMHO)

For the really deep shine finishes you can wet sand with ever finer grits (e.g. 1200->1500->2000) before using something like Farecla G3. I've managed to get almost mirror like finishes with that process, but it's a lot of work.

I should add that my knowledge and experience is from the car body repair world, brother in law has a body shop.
Makes life nice and easy for most of my projects as I get to use the workshop as my own.

I'm not jealous. Oh, wait. Yes. I'm jealous. Very jealous.