Priming MDF boxes

[Ricky]

How do commercial manufacturers deal with this?

I have a pair of tiny audax based bookshelf speakers that are made from 12mm MDF. I've had the drivers out and I've look at the box construction - butt joints with round over. The finish applied is a matt black but I can't for the life of me see a single joint. The finish is near indestructible, they've taken a couple of knocks and the MDF dents but I'm quite sure if this paint was used on a tough surface such as steel then you'd have to take a screw driver and some elbow grease to really damage the stuff.

Commercially, the finish on MDF goes like this.

1). Sand MDF surface thoroughly. I use 120 grit and 320 grit on a D/A sander. Ensure all joints are tight. No cracks or gaps. If so, use P38 filler SPARINGLY.

2). Apply 1 coat of MDF Sealer (solvent based). Allow to dry and then sand with 320 paper. Apply 2nd coat but this time, lightly sand with either 80 or 100 grit paper to key surface.

3). Apply 3 coats of primer. Pre-cat is the easiest, rubbing down in between coats.

4). Apply top coat (s).

Ricky.

 

[richie00boy]

Perfectly safe to put body filler over cured polyurethane. That's how I do things - prime, repair then paint. I never attempt repairs (other than sanding) before priming.

I had always been told to put filler on first, as if you put it on paint it can sink. Certainly seems to have done that on some repairs I've seen.

2). Apply 1 coat of MDF Sealer (solvent based). Allow to dry and then sand with 320 paper. Apply 2nd coat but this time, lightly sand with either 80 or 100 grit paper to key surface.

3). Apply 3 coats of primer. Pre-cat is the easiest, rubbing down in between coats.

So what's this magic MDF Sealer then?

And what's pre-cat?

 

[MJL21193]

How do commercial manufacturers deal with this?
... you'd have to take a screw driver and some elbow grease to really damage the stuff.

My guess is a high build, 2 part industrial coating on those bookshelves. Not unlike the product from Sherwin-Williams that I posted earlier.

As for overcoming the glue line/butt joint issue, as I've said, I don't have that problem. The surround speaker a few posts back (the one that I put the waterbased Varathane on) has no visable seams, and that was finished years ago.

I think it all goes back to the moisture content before finishing.

Ant did you try the waterbased samples yet? Even though I get great results with what I use, I'd like to switch to spraying waterbased (safer) products. It makes me nervous spraying highly flammable stuff - have visions of me being the human torch.

 

[MJL21193]

I had always been told to put filler on first, as if you put it on paint it can sink. Certainly seems to have done that on some repairs I've seen.



So what's this magic MDF Sealer then?

And what's pre-cat?

Rich,
Not sure what you mean by "sink". Autobody filler has strong solvents in it (styrene) that will desolve some paints and clears, such as lacquer. It's not strong enough to attack cured polyurethane though.

Ricky sounds like he knows what he's talking about.
There are specific MDF sealer available, but the main thing is stability, not to have its solvent absorbed and it's solids left on the surface.

Pre-cat is pre catalysed. Means it's a tough 1 part coating.

 

[Ricky]

Hi all.

The reason I 'appear' to know what I'm talking about is because I am doing a design right now using the very principles we are discussing.

MJL is right in his post about water based products. My only concern is this. MDF, when it gets wet, expands on it's ends due to water reaction with the glue content of the board, causing it to 'flare'. That is to say expand. These water based solutions are also very heavy and, whilst they don't hang in the air after being sprayed, they tend to stick to everything else. Also, you need a top notch expensive spray gun to apply these finishes successfully and good extraction. A gravity gun is better than a suction cup gun.

I'm going for a gloss black finish similar to Wilson Audio's.

One more word about paint. I have been warned away from painting because it can crack after time. So far, loada rubbish. The secret is in the preparation. I don't put the top coats on myself so I use a guy that's been spraying Bentleys and Mercs for the last ten years or so.

All the best.

Ricky.

 

[sploo]

Is the finish on those little boxes textured at all Ant? Could it be this? http://www.aldcroftadhesives.com/productdetails.php?ProductID=92

That looks like it could be interesting. The technical sheet link doesn't work, so I've sent them a mail asking for more details.

...Sploo, if you had all those clamps, why didn't you use them? 🙁
I only recommended building your own because I thought you didn't have any...

I had problems with the layers slipping against one another when trying to use clamps - hence my plan to build some form of press.

Ironically, this was mainly due to using lots of glue!

...Perfectly safe to put body filler over cured polyurethane. That's how I do things - prime, repair then paint. I never attempt repairs (other than sanding) before priming.

I take it you'd apply another coat of primer over the filler? I'd expect the filler would behave a little like MDF (porous) if painted over without being sealed (PP100 filler recommends being primed before painting).

Commercially, the finish on MDF goes like this...

Ricky, seconding Rich's post - could you tell us the products that are being used here (brand names etc.)?

 

[pinkmouse]

...I had problems with the layers slipping against one another when trying to use clamps - hence my plan to build some form of press...

Sprinkle a tiny amount of sand on the layers as you glue them up. 😉

 

[ShinOBIWAN]

Sprinkle a tiny amount of sand on the layers as you glue them up. 😉

Good tip, I'd imagine the sand would get crushed letting the the gap in the joint be comparable to no sand. This is providing you didn't use a lot.

Another method for laminations I use is to press the pieces together using just the pressure from your hands/bodyweight, align the pieces and then leave for about 15 minutes. After this time the glue is going off and you can clamp without much movement.

 

[ShinOBIWAN]

I had always been told to put filler on first, as if you put it on paint it can sink. Certainly seems to have done that on some repairs I've seen.

The filler I currently use reacts with cellulose paints. So the primer I use would be a no-no if applying the filler over that.

So what's this magic MDF Sealer then?

Yes please, tell!

 

[ShinOBIWAN]

Is the finish on those little boxes textured at all Ant? Could it be this? http://www.aldcroftadhesives.com/productdetails.php?ProductID=92

Might well be. Its pretty smooth overall put there's a definite texture to it that feels and looks more like slight orange peel than anything.

 

[Ricky]

Hi.

I think we need to stand back and look at the whole cabinet building thing.

Firstly, if you're going to build good strong cabinets, you need to use mitre joints. Why?. Because a 45 degree mitre on an 18mm board gives you a joint area of approx 25mm's. Secondly, there is no "end grain" showing when you put the cabinet together so, if you think about it rationally, there is nothing to "sink". Each panel needs to be mitred on all 4 sides. Mitring is easy. I'll explain, for those of you that are a little woodwork shy.

Let's say that you're cabinet is 650mm tall, 350mm wide and 250 deep. Like I said, I'm gonna keep this simple. For this cabinet, you need 2 off 650mm x 350mm, 2off 650mm x 250mm and 2off 350mm x 250mm for tops and bottoms.

Cut the panels square edged first. Then, if you're working on the 650mm, DEDUCT the thickness of the board!!. If it's 18mm, set the rip fence on the saw to 632mm ( 650mm - 18mm ) THEN tilt the saw blade to 45 degrees. Put the panel thro twice and repeat for all the other dimensions.

If the saw is true (check it with a square) all of the mitres should go together. Place the 650mm x 350mm and 250mm x 350mm on the bench in a long line (650,250,650,250) and match all the panel widths up. Offer all the edges to one another and tape them using 3M or similar masking tape. When this is done, place a thin board underneath and "flip" the panels over. Now dry fold the cabinet sides and top and bottom using the masking tape as a hinge. Unfold, put glue in the joints and fold together again. Turn thro 90 degrees and fit the front and back.

One word of caution. Use a chalk-free PVA or else it could separate in the cold weather. After all this, sand to suit and seal. If you've glued the joints properly, there's very little to fill hence, no sinking before primer.

To all of you that are already experienced with this, my profound apologies. To those of you that aren't, this is the best way to go but, do me a favour. DON'T chop yer fingers off!!!. Get a sawmill to do it if you are unsure in any way with wood machines. They can bite, and hard!!!. You have been warned.

No butt joints or half-lap (rebated) joints unless you use screws as well. It just doesn't work.


Ricky.

 

[sploo]

...Firstly, if you're going to build good strong cabinets, you need to use mitre joints...

I don't totally agree with that. Whilst it's better than simple butt joints, I would think a rebate/dado type join is the strongest, and, just has the larger surface area for the joint.

Also, you really need to get your mitres absolutely perfect for there to be a perfect join line - with a rebate you can trim/sand. You can of course trim/sand a non perfect mitre join, but that would result in end grain, so you don't gain anything over a rebate.

The final reason mitres aren't always useful - look at the stuff Ant is doing; layers of sheets with angled cuts (exposing end grain). My stuff uses a slightly different lamination technique, but equally exposes lots of end grain.

Here's an example - complete with ropey sealing job using epoxy, finding it was a PITA to sand inside the unit, realising I'd made some mistakes with the CNC process so the parts needed loads of sanding etc. etc.

http://spikyfish.com/CDRack/CNC.jpg
http://spikyfish.com/CDRack/With_CDs.jpg
http://spikyfish.com/CDRack/Epoxy_Coated.jpg
http://spikyfish.com/CDRack/Finished_front.jpg
http://spikyfish.com/CDRack/Finished_angle.jpg


Quick update - I've had a response from Vandersteen. Basically, yes, we're laminating. Yes, it was difficult to find a process. No, we're not going to tell you how.

I've got the datasheet from aldcroftadhesives about their speaker coating, and asked them about the specific stuff we're doing. I'll see if they think it's suitable.

I'm also following up one other lead, which I'll discuss if/when I hear something.

 

[ShinOBIWAN]

Hi Ricky,

Agree with most of and mentioned using differing joints to minimise the problem in this thread:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=100835&highlight=

However:

No butt joints or half-lap (rebated) joints unless you use screws as well. It just doesn't work.

It would appear that way but John is doing fine and the little budget Audax standmount I mentioned early is another example. So its possible to get good results. Unfortunately it appears to be eluding us UK based folks.

 

[ShinOBIWAN]

Originally posted by sploo The final reason mitres aren't always useful - look at the stuff Ant is doing; layers of sheets with angled cuts (exposing end grain). My stuff uses a slightly different lamination technique, but equally exposes lots of end grain.

Yes it would have been a headache and inferior to have used mitres to form the baffle:

An externally hosted image should be here but it was not working when we last tested it.

I did try to minimise the problem of joint issues to just the facets by skinning the rest with 9mm. You should be able to make that out in the photo.

Then the facets with the joints were veneered and then sprayed over. About 2 and half month on and still no joints showing but its a PITA to veneer and then spray over that. Its a lot of work really.

Quick update - I've had a response from Vandersteen. Basically, yes, we're laminating. Yes, it was difficult to find a process. No, we're not going to tell you how.

Well Vandersteen ain't getting any of my money now 😀

 

[ShinOBIWAN]

Ant did you try the waterbased samples yet? Even though I get great results with what I use, I'd like to switch to spraying waterbased (safer) products. It makes me nervous spraying highly flammable stuff - have visions of me being the human torch.

Won't be able to give it a go for another couple of months yet. Whenever I get the paint kit out its to spray something for that ongoing project of mine.

I'll certainly be giving them a go at somepoint though - no other choice really, thanks to the good old EU banning half the paints I use next year 🙄

 

[Ricky]

I think I'm gonna shut up and keep my opinions to myself. We were talking about MDF, not hardwood laminates. Of course the way stepped baffles and the like get laid up is different and, conversely, the way they are sealed and treated is different. Only an absolute pillock would treat hardwood with sealer the way I stated.

Hae seen the way the hardwood sculpted baffles are in the photos preceeding this post. Gorgeous. If anyone was to treat that the way I said deserve to hae their nuts shot off.

Like I said, I'm gonna keep quiet. After all, what do I know. I'e only bin doin this for 20 years with 4 5* revviews under me belt.

Ricky.

 

[ShinOBIWAN]

I think I'm gonna shut up and keep my opinions to myself. We were talking about MDF, not hardwood laminates. Of course the way stepped baffles and the like get laid up is different and, conversely, the way they are sealed and treated is different. Only an absolute pillock would treat hardwood with sealer the way I stated.

Hae seen the way the hardwood sculpted baffles are in the photos preceeding this post. Gorgeous. If anyone was to treat that the way I said deserve to hae their nuts shot off.

Like I said, I'm gonna keep quiet. After all, what do I know. I'e only bin doin this for 20 years with 4 5* revviews under me belt.

Ricky.

Er, Ricky calm down mate. Not quite sure who or what was said to prompt that response? Everyone agreed with virtually all you've said in this thread.

I'm not sure where the hardwood laminates reference came in either, nobody has used this in here. This is an MDF only discussion we've been having. Everything show/discussed is MDF based.

Your input is definitely appreciated, so please stick around.

 

[sploo]

Ricky - you've contributed some interesting, well written, and informative posts in this thread. Ant is mostly agreeing with you, I differ on the approach to joints, but hey, it'd be a boring world if we agreed on everything.

Mostly I was pointing out that the style of boxes we're doing results in lots of end grain, so the joint type has less bearing on the outcome.

I'm still waiting, and very much interested, to know what products were being used to seal and prime the MDF in your post (#181). In short - please continue to post.

The only thing that's got me a bit confused is the reference to hardwood laminates. 😕

 

[MJL21193]

Sprinkle a tiny amount of sand on the layers as you glue them up. 😉

Excellent tip. I usually fire an inconspicuous brad in here or there.
🙂

 

[MJL21193]

...I would think a rebate/dado type join is the strongest, and, just has the larger surface area for the joint.

Rabbit (rebate/dado) joint does give more surface area. Also minimizes the end grain exposure. But still, not much good on a 18 sided box.😀
BTW very nicely done on the cd rack Sploo.

...but hey, it'd be a boring world if we agreed on everything.

I agree. I'm usually the one shaking things up!