How do you cut it or round the edges and "not break the surface"? I agree on the flat rolled surface this is key, but there is always an edge, and it is the edges that present the problems.
Yes, that's why it needs to be completely sealed everywhere, which is a real pain.
Here is how I do it. I am not very good at it but it's getting there.
I have a small air compressor (1.5hp/4,5 gallons) and bought a gravity fed HVLP spary gun.
Once the box is built, I give it 2-3 coats of any brand and any color INSIDE! 🙂
This will seal any dust and will also prevent somewhat humidity to be absorbed by the bare mdf. You may ask since I have a spray gun, why use cans, well, the gun takes quite a bit of time to clean after a spray job! and I am lazy!
All my speakers are rounded on the edges with a router. This will leave the edges quite rough. I sand them with #120 sand paper and then I coat the edges with 20 minute epoxy glue applied with a small throw-away brush. One coat will do usually.
Then the trick is to apply car body putty with a squeegee all over the box. Sand lightly first with #120 to remove the streaks that you will do with the squeegee, with a random orbital sander but not the edges, just the flat surface. Go lightly.
Then I use #400 grit for the final sanding. I do the edges with #400 by hand, never with a sander!
One coat of gray primer will let you see where your finish stands at the moment, probably quite well at this point. I use what is called filling primer. This primer has a larger solid content and will fill small cracks and pinholes.
Paint the color you prefer and remember many light coats is better than one thick one!
I use #600 worn sand paper between each coat and I finish with a clear coat.
I am not yet at the 'piano' finish yet, but now I get a rather good presentable finish consistently 🙂
Luc
I have a small air compressor (1.5hp/4,5 gallons) and bought a gravity fed HVLP spary gun.
Once the box is built, I give it 2-3 coats of any brand and any color INSIDE! 🙂
This will seal any dust and will also prevent somewhat humidity to be absorbed by the bare mdf. You may ask since I have a spray gun, why use cans, well, the gun takes quite a bit of time to clean after a spray job! and I am lazy!
All my speakers are rounded on the edges with a router. This will leave the edges quite rough. I sand them with #120 sand paper and then I coat the edges with 20 minute epoxy glue applied with a small throw-away brush. One coat will do usually.
Then the trick is to apply car body putty with a squeegee all over the box. Sand lightly first with #120 to remove the streaks that you will do with the squeegee, with a random orbital sander but not the edges, just the flat surface. Go lightly.
Then I use #400 grit for the final sanding. I do the edges with #400 by hand, never with a sander!
One coat of gray primer will let you see where your finish stands at the moment, probably quite well at this point. I use what is called filling primer. This primer has a larger solid content and will fill small cracks and pinholes.
Paint the color you prefer and remember many light coats is better than one thick one!
I use #600 worn sand paper between each coat and I finish with a clear coat.
I am not yet at the 'piano' finish yet, but now I get a rather good presentable finish consistently 🙂
Luc
And you have not even found the real problem, if you used PVA glue, it shrinks over time and the seams will show up through the paint in a year.
My current test build is to sand square with a belt sander after I rounded ALL 12 EDGES, then seal with several coats of polyurethane resin, squeegeed on. I then bake at 170 for 2 hours to get the resin as hard as it is usually after 6 months. This lets me sand a stable plastic substrate I can wet sand. I fill the pinholes, prime and paint, it should come out as nice as my TVR body did. Or better I hope. For automotive finish, use automotive paint. Dupont is what my local jobber carries.
Normal auto filler is too thick. Evercoat makes a thinner filler that does much better. You can thin regular Bondo with poly resin. Lastly use the red spot putty for pinholes. It is my hope the resin sealer will prevent the seams from coming though.
My current test build is to sand square with a belt sander after I rounded ALL 12 EDGES, then seal with several coats of polyurethane resin, squeegeed on. I then bake at 170 for 2 hours to get the resin as hard as it is usually after 6 months. This lets me sand a stable plastic substrate I can wet sand. I fill the pinholes, prime and paint, it should come out as nice as my TVR body did. Or better I hope. For automotive finish, use automotive paint. Dupont is what my local jobber carries.
Normal auto filler is too thick. Evercoat makes a thinner filler that does much better. You can thin regular Bondo with poly resin. Lastly use the red spot putty for pinholes. It is my hope the resin sealer will prevent the seams from coming though.
I watched lots of youtube videos prior to starting and they all seemed to agree that priming, painting, and then sanding with progressively finer and finer sandpapers (wet sanding at the 1000 grit level), and then hitting it with car rubbing compound and car wax would get the gloss that I am looking for. If it doesn't work out I can always sand it back down to wood and try again
😉
filler, sanding, filler, sanding, filler ....
primer, sanding, filler, sanding, filler, sanding, primer, sanding, filler, sanding, priming, sanding ... that will do 😀
I watched lots of youtube videos prior to starting and they all seemed to agree that priming, painting, and then sanding with progressively finer and finer sandpapers (wet sanding at the 1000 grit level), and then hitting it with car rubbing compound and car wax would get the gloss that I am looking for. If it doesn't work out I can always sand it back down to wood and try again
Yes, that is basically correct, but here is the rub. One pinhole in the finish and the MDF will blowup like a balloon when wet sanding. And you simply cannot get a smooth enough finish unless you wet sand. Sure it can be done, but it is not easy and for a first timer its going to be very very difficult. I gave up on MDF and PVA glue for all of the reasons given above. I now use polyurethane boards and epoxy and have none of these problems. Unfortunately these boards are an order of magnitude more expensive than MDF, but the savings in time and the much higher quality of the final assembly is well worth it.
Just an idea I have got and not tried but it may produce a very nice finish.
Build, sand, putty and prime the box. Then put just one coat of white paint for a light finish, and dark grey or even black if you want a dark colour.
Be extremely maniacal about removing all the dust on the surface and coat the whole thing with heat-shrink plastic film covering used in radio control airplane modeling.
You can have satin or glossy finish in many colours! 🙂
The film is removed from a backing and applied on the box. There should be only one seam that will be slightly apparent. You can make this seam in a corner or underneath to hide it somewhat. Then either use a clothes iron or a heat gun and run it over the film. It will shrink and the glue that was just tacky will melt and adhere to the surface beneath. Voilà! Instant finish! 🙂
There are many videos on you tube explaining how to proceed to cover a model airplane, I guess the same technique can be applied for a speaker box.
The reason you should put at least on coat of paint before covering is that the films are not completely opaque and differences in colour underneath will show.
The film is sold in hobby shop, try search for Tower Hobbies, they have this stuff.
Luc
Build, sand, putty and prime the box. Then put just one coat of white paint for a light finish, and dark grey or even black if you want a dark colour.
Be extremely maniacal about removing all the dust on the surface and coat the whole thing with heat-shrink plastic film covering used in radio control airplane modeling.
You can have satin or glossy finish in many colours! 🙂
The film is removed from a backing and applied on the box. There should be only one seam that will be slightly apparent. You can make this seam in a corner or underneath to hide it somewhat. Then either use a clothes iron or a heat gun and run it over the film. It will shrink and the glue that was just tacky will melt and adhere to the surface beneath. Voilà! Instant finish! 🙂
There are many videos on you tube explaining how to proceed to cover a model airplane, I guess the same technique can be applied for a speaker box.
The reason you should put at least on coat of paint before covering is that the films are not completely opaque and differences in colour underneath will show.
The film is sold in hobby shop, try search for Tower Hobbies, they have this stuff.
Luc
I built radio control airplanes when I was younger so I know exactly what you are talking about. Not sure I want to try to do a speaker with it though.....lol
I have thought about this as well, but it seems like a bigger speaker with no real "edges" could be a problem.
jerseyguy1996 ... couple things here. You don't mention where you bought the mdf ... the home centers will have the worst storage criteria out there. For any panel to remain flat within the 0.005" manufactured tolerances, they need to be stored that way. Same goes for you after purchase and during machining.
Looking at the sanding pattern you've developed on the sides I wonder if your interior bracing is at fault here as well. Were the panels used inside perfectly square with parallel edges? A 1/32" or even a 1/64" variance inside will cause the exterior panel to deflect. If you have to have a "perfectly" flat surface you should be checking for that during assembly ... adjusting the interior parts accordingly. Even then, you will have variances.
What equipment/sanding techniques are you using to come to the conclusion that the panels aren't flat enough? No offense, but I've seen people that couldn't sand to save their lives.
Looking at the sanding pattern you've developed on the sides I wonder if your interior bracing is at fault here as well. Were the panels used inside perfectly square with parallel edges? A 1/32" or even a 1/64" variance inside will cause the exterior panel to deflect. If you have to have a "perfectly" flat surface you should be checking for that during assembly ... adjusting the interior parts accordingly. Even then, you will have variances.
What equipment/sanding techniques are you using to come to the conclusion that the panels aren't flat enough? No offense, but I've seen people that couldn't sand to save their lives.
Yes, mentioned a few times the dark mdf is the worst of all.
A step up is the lighter color shelving board - however!! box stores just look for the best deal vs. availability and have seen the lighter color mdf in 4x8's on the shelves, time to time.
Cabinet shops don't mess with any of this, for a few dollars more, they'll order MEDEX, which is is a nicer grade all around.
Rolling primer - this never worked for me and left traces of roller marks, orange peal; more work.
As TVR mentioned, don't hurry to finish mdf projects, let them adjust to house humidity before going to far.
A step up is the lighter color shelving board - however!! box stores just look for the best deal vs. availability and have seen the lighter color mdf in 4x8's on the shelves, time to time.
Cabinet shops don't mess with any of this, for a few dollars more, they'll order MEDEX, which is is a nicer grade all around.
Rolling primer - this never worked for me and left traces of roller marks, orange peal; more work.
As TVR mentioned, don't hurry to finish mdf projects, let them adjust to house humidity before going to far.
Great thread! I'll join into the "Oh, I thought it was just me" chorus. Eventually I just gave up, rolled on house paint and figured it was ugly enough. 🙁
Over the last decade I have found painting to be the hardest thing to get right. I can do it now, but what a pain. That's why I knock off so much to ship them unfinished. A couple of customers have had them pained by a pro - at significant expense I might add - and they were absolutely fantastic. It takes a big facility with a paint booth to get that kind of finish however.
A friend of mine was President of Boston Acoustics and he said that finish had by far the highest reject rate of any flaw.
A friend of mine was President of Boston Acoustics and he said that finish had by far the highest reject rate of any flaw.
To succeed with painting MDF you have to use the right type of MDF. As mentioned the dark kind sucks, literally. Cabinet makers and woodshops use a different kind of MDF when making furniture that is to be painted. This is what I have used: Fibralac, but there's a multitude of similar products depending on where you are.
Also, one of the reasons commercial MDF products aren't painted to high gloss but rather satin/semi gloss is probably due to the fact that the demand for a perfectly prepped surface make it dramatically more expensive.
If you want a decent result easy you should use MDF made for routing and low water & paint absorption, then paint it with some 2comp acid curing paint system.
Also, one of the reasons commercial MDF products aren't painted to high gloss but rather satin/semi gloss is probably due to the fact that the demand for a perfectly prepped surface make it dramatically more expensive.
If you want a decent result easy you should use MDF made for routing and low water & paint absorption, then paint it with some 2comp acid curing paint system.
It's definitely due to the internal bracing and also I sanded with a random orbit sander which I think bit into the sides and corners more than the center.
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