This may seem off but I use an automotive repair product, Bondo, in both woodworking and painting. It dries rapidly into an extremely tough impervious substance. It does take a power sander to smooth and is designed to take paint or other finishes well. You will not see anything through it. It was designed for exterior auto repair and to be seamlessly painted over. It is also frequently used by painters to repair exterior rotten or gouged wood prior to painting.
I've used Bondo for about 45 years, first for body and fender work, then on wood, MDF, and etc. The original formula used regular fiberglas resin with talc mixed in for bulk and it was very hard to sand. There is a variety of "Bondo Lite" that uses microspheres or microballoons for the bulking agent and is an order of magnitude easier to sand. It really speeds up the work and yet provides a surface that is smooth enough for primer to be applied. If your Bondo store has several types or brands, just heft the cans and use the lightest product. Once you've tried it, you'll never want to go back.
Best Regards,
TerryO
Best Regards,
TerryO
TerryO's "Bondo" solution has merit.
When ur doing Bondo work, and you want to fine finish minor imperfections, you get "automotive glazing putty". It is one step, spreads very smooth and sands just fine. Solvent based. Comes in a tube. Ought to work well on edges of MDF, I suspect.
Before they got rid of nitrocellulose, the equivalent stuff was "Nitro-Stan". I presume a nitrocellulose base with a solvent... don't think you can get that anymore. :-( That stuff worked better than the new stuff.
Keep in mind that MDF likes to expand in the thickness dimension due to moisture, while it seems to not expand in the other dimensions (at least not quite as much) - this due to ambient moisture. A case could be made for "aging" MDF... sooooo, the advice earlier in the thread about coating/painting the MDF inside and out is not bad advice, IF edges are visible.
I think this is a major reason that higher quality MDF work uses mitred and biscuited corners, not butted corners.
_-_-bear
PS. I have not tried the "PVA glue" method, but it might work fine with a few coats, lightly sanded between until you get a solid smooth surface...
When ur doing Bondo work, and you want to fine finish minor imperfections, you get "automotive glazing putty". It is one step, spreads very smooth and sands just fine. Solvent based. Comes in a tube. Ought to work well on edges of MDF, I suspect.
Before they got rid of nitrocellulose, the equivalent stuff was "Nitro-Stan". I presume a nitrocellulose base with a solvent... don't think you can get that anymore. :-( That stuff worked better than the new stuff.
Keep in mind that MDF likes to expand in the thickness dimension due to moisture, while it seems to not expand in the other dimensions (at least not quite as much) - this due to ambient moisture. A case could be made for "aging" MDF... sooooo, the advice earlier in the thread about coating/painting the MDF inside and out is not bad advice, IF edges are visible.
I think this is a major reason that higher quality MDF work uses mitred and biscuited corners, not butted corners.
_-_-bear
PS. I have not tried the "PVA glue" method, but it might work fine with a few coats, lightly sanded between until you get a solid smooth surface...
sploo said:
Great thread, since I need to make the edges of the MDF invisible on my BIB I am building.
I will have to see what kind of epoxy is at Homedepot or Lowes here in US. Anyone have recommendations? I plan to finish with latex paint followed by polyurethane, will this work?
thanks,
gychang
Re: Re: Priming MDF boxes
gychang,
I've used epoxy on several boats both as an adhesive and as a coating and sealer. While I haven't priced it for a few years now; System 3, made here in the NW for boatbuilding, was as good, IMO, as West Systems products and used to be a lot cheaper. They had one type (formula) of epoxy that wasn't intended as an adhesive, but rather as a hard clearcoat and sealer. It was a lot thinner and took longer to set up, so it penetrated extreamly well and was often used to treat spots of dry rot. It just soaked in and then consolidated the rot when it hardend, saving what otherwise might have been an expensive repair. This product, or one similar, would be the one I would use for the raw edges of MDF.
However YMMV.
Best Regards,
TerryO
gychang said:
gychang,
I've used epoxy on several boats both as an adhesive and as a coating and sealer. While I haven't priced it for a few years now; System 3, made here in the NW for boatbuilding, was as good, IMO, as West Systems products and used to be a lot cheaper. They had one type (formula) of epoxy that wasn't intended as an adhesive, but rather as a hard clearcoat and sealer. It was a lot thinner and took longer to set up, so it penetrated extreamly well and was often used to treat spots of dry rot. It just soaked in and then consolidated the rot when it hardend, saving what otherwise might have been an expensive repair. This product, or one similar, would be the one I would use for the raw edges of MDF.
However YMMV.
Best Regards,
TerryO
Sploo,
I was wondering if you could post pictures of the cracks developing in the other samples? Your black epoxy sample looks very nice. I am in the midst of a project finishing up with wood hardener, but my speakers were already sealed up, so I can't cover all the surfaces. I'd like to see an example of what I'm in for...
apologies if a picture like I request has already been posted, I've read thought this thread over a few different sessions, and may have missed it.
-A
I was wondering if you could post pictures of the cracks developing in the other samples? Your black epoxy sample looks very nice. I am in the midst of a project finishing up with wood hardener, but my speakers were already sealed up, so I can't cover all the surfaces. I'd like to see an example of what I'm in for...
apologies if a picture like I request has already been posted, I've read thought this thread over a few different sessions, and may have missed it.
-A
I believe Shinobiwan has already confirmed that sealing with thinned PVA is not any good.
Great stuff Sploo. I'm not fussed that lines appear in the epoxied ones at first as, as you say, this can be flatted out. Looks like I'll be buying some epoxy in then. I'll be trying cheaper stuff than West Systems so we'll see how it goes. But I'm not sure I'll be finishing in such a high gloss as the first speakers to get this treatment will be some transportable party speakers.
Great stuff Sploo. I'm not fussed that lines appear in the epoxied ones at first as, as you say, this can be flatted out. Looks like I'll be buying some epoxy in then. I'll be trying cheaper stuff than West Systems so we'll see how it goes. But I'm not sure I'll be finishing in such a high gloss as the first speakers to get this treatment will be some transportable party speakers.
Application and technique are everything in finishing.
The thinned PVA method may work just fine, or not work, depending on the application and technique used.
Seems to me the thinned PVA is intended to penetrate the surface slightly. To hide a crack or seam the surface would have to be both filled, flat and continuous. That implies multiple layers/applications, with fine sanding between. The result being a thin continuous layer of PVA with no surface deviation over the seam/glue joint, whatever...
gychang -
Polyurethane over Latex paint?
Probably not a great idea.
Polyurethane over poly paint? better idea.
No matter what I'd test first... my concern is that the "Latex" will not bond well with the Polyurethane (presume varnish here), and maybe will react some. Long term is an issue if this is a nicely done piece of wood work...
Dunno about the water based poly over "latex"...
Seems to me that applying a thin solvent based paint over a thin solvent based primer with adequate dry time followed by a poly gloss or semi gloss finish would be a better plan... housepaint tends to be very heavy and thick. If you spray, maybe not so bad, but then use gloss paint??
_-_-bear
The thinned PVA method may work just fine, or not work, depending on the application and technique used.
Seems to me the thinned PVA is intended to penetrate the surface slightly. To hide a crack or seam the surface would have to be both filled, flat and continuous. That implies multiple layers/applications, with fine sanding between. The result being a thin continuous layer of PVA with no surface deviation over the seam/glue joint, whatever...
gychang -
Polyurethane over Latex paint?
Probably not a great idea.
Polyurethane over poly paint? better idea.
No matter what I'd test first... my concern is that the "Latex" will not bond well with the Polyurethane (presume varnish here), and maybe will react some. Long term is an issue if this is a nicely done piece of wood work...
Dunno about the water based poly over "latex"...
Seems to me that applying a thin solvent based paint over a thin solvent based primer with adequate dry time followed by a poly gloss or semi gloss finish would be a better plan... housepaint tends to be very heavy and thick. If you spray, maybe not so bad, but then use gloss paint??
_-_-bear
gychang said:
On the first test piece, I sealed the sides and top. On the second, everything.
I'm working on a 'proper' use of the technique at the moment (a CD shelf/cabinet 'thingy') and again, I'm sealing all faces - cut and uncut.
AdamThorne said:
I believe Shinobiwan has a pic showing the problem on one of his speakers, but essentially, it's as if the joins have parted very slightly, and the paint surface has 'dropped' into the gap.
Now... bad news... since taking that photo a few days ago, I've just noticed that one part of the second block has developed some small lines.
The thing that mystifies me is that the first block (done much earlier, and not completely coated) looks just fine. I did however have a much thicker coat of epoxy on that one, so maybe it needs to be slapped on with a trowel...
richie00boy said:
Yea, PVA will (usually) be thinned with water. Water + MDF = BAD!
I was chatting to a friend who builds planes, and he noted that epoxy does shrink over time. His suggestion was that I leave the sanding of the epoxy for a few days - i.e. don't sand and paint just a day or so later.
It would be nice to find something cheaper than the West System stuff. I have found it goes a long way for the money, but it appears that laying it up thick may be required (hence a cheaper product would be good).
I did find the West guys very helpful when I originally spoke to them about the covering requirements. I'll knock out another email and see what they think with regard to these problems.
Member
Joined 2003
Member
Joined 2003
MDF moisture the problem
I'm not an expert in this area, but I have seen posts by some
experts. There's a guy over on the PartsExpress forum with
a unique solution. The guy's user name is Aaron Hero.
Sorry, I can't find his original posts to show the solution.
He has built a rig to force hot air into a MDF box - IIRC, he
uses some infrared lights in a box lined with concrete
Hardibacker, and a small 120 V fan. This is connect to the box
under construction for 24 - 48 hours, and the 140-160 F degree
hot air dries out the box completely from the inside out.
Once it's completely dry, he uses high-build automotive primer
and automotive lacquer for a finish. Says he's never had a problem
with seams or cracking later on.
Sorry I can't provide more details....
HTH,
-Don
I'm not an expert in this area, but I have seen posts by some
experts. There's a guy over on the PartsExpress forum with
a unique solution. The guy's user name is Aaron Hero.
Sorry, I can't find his original posts to show the solution.
He has built a rig to force hot air into a MDF box - IIRC, he
uses some infrared lights in a box lined with concrete
Hardibacker, and a small 120 V fan. This is connect to the box
under construction for 24 - 48 hours, and the 140-160 F degree
hot air dries out the box completely from the inside out.
Once it's completely dry, he uses high-build automotive primer
and automotive lacquer for a finish. Says he's never had a problem
with seams or cracking later on.
Sorry I can't provide more details....
HTH,
-Don
Re: MDF moisture the problem
Interesting. I had a look at the Tetrosyl website, and they have various fillers and puttys. Couldn't spot anything that looked quite right though.
£8 for a litre would be an excellent price. Much cheaper than West stuff.
I think I may have seen this. I saw a page with a very nice shaped, grey (presumably primed) box, with a pipe going to one of the driver holes, and a heater box supplying warm air.
It's a nice idea if the movement problems are due to the MDF drying out. Of course, if your climate is fairly humid, then you might just be pumping warm wet air through the box...
richie00boy said:
Interesting. I had a look at the Tetrosyl website, and they have various fillers and puttys. Couldn't spot anything that looked quite right though.
£8 for a litre would be an excellent price. Much cheaper than West stuff.
rj45 said:
I think I may have seen this. I saw a page with a very nice shaped, grey (presumably primed) box, with a pipe going to one of the driver holes, and a heater box supplying warm air.
It's a nice idea if the movement problems are due to the MDF drying out. Of course, if your climate is fairly humid, then you might just be pumping warm wet air through the box...
Hi Sploo
Good to see your still at it. The whole thing is rather unpredictable and there's a waiting game to be played whilst you hope the lines don't show - that part I don't like. It does sound like the epoxy could be a winner but I'm sure it would also be somewhat dependant on the type used. For example the polyester resin didn't do much good.
For reference, the veneering method I mention in another post is still going good a ~7 weeks later. By this point if lines haven't start to show slightly I don't think they every will TBH.
If the epoxy method can be made to work and reliably thats certainly easier and quicker than veneering for sure.
Please keep us updated.
Good to see your still at it. The whole thing is rather unpredictable and there's a waiting game to be played whilst you hope the lines don't show - that part I don't like. It does sound like the epoxy could be a winner but I'm sure it would also be somewhat dependant on the type used. For example the polyester resin didn't do much good.
For reference, the veneering method I mention in another post is still going good a ~7 weeks later. By this point if lines haven't start to show slightly I don't think they every will TBH.
If the epoxy method can be made to work and reliably thats certainly easier and quicker than veneering for sure.
Please keep us updated.
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