Hey guys. No matter how much I try and sand, I cannot get rid of edge markings or panel joints when i go to prime. Is it my sander? I had a black and decker one, and just now bought http://www.ridgid.com/Tools/R2600/index.htm to replace it. Am I using the right type of tool, or is it something I am doing wrong? Thanks for any help!
The problem I have is the end grain absorbing the paint at a different rate than the face of the MDF. Here is what I've found works the best for me:
If there is any gap at all I use bondo to fill it. I just use the cheap automotive stuff that comes with the tube of hardener. This stuff doesn't sand out of the gap as easy as wood filler does. Just make sure you sand with either a power sander or use a block.
Sand the edge until you cant feel anything but a smooth surface. If you can feel it in the slightest, you will be able to see it.
Next, I take some white glue (I used elmers because that's what I had) and dilute it with some water. Brush it over the entire outside of the cabinet with a foam brush. Once it's dry, sand it down with 400 grit paper and you'll end up with a super smooth surface that will take paint evenly.
Although I haven't tried it, I've heard that sanding sealer will work as good as the diluted glue method. Good luck!
If there is any gap at all I use bondo to fill it. I just use the cheap automotive stuff that comes with the tube of hardener. This stuff doesn't sand out of the gap as easy as wood filler does. Just make sure you sand with either a power sander or use a block.
Sand the edge until you cant feel anything but a smooth surface. If you can feel it in the slightest, you will be able to see it.
Next, I take some white glue (I used elmers because that's what I had) and dilute it with some water. Brush it over the entire outside of the cabinet with a foam brush. Once it's dry, sand it down with 400 grit paper and you'll end up with a super smooth surface that will take paint evenly.
Although I haven't tried it, I've heard that sanding sealer will work as good as the diluted glue method. Good luck!
Okay, basically, it feels like theres no blemishes, but there obviously are. The way that I prep for painting is to sand down the enclosure with 60,80, 120 and then 220 grit sandpaper on on the sander. Then, I take some titebond and mix it with water and apply about 8-10 coats of gluewash to seal the cabinets. Sand down with 220 grit and then 320. It feels like glass at this point. However, as soon as I start spraying my primer, I immediately see the outline of the panels through the paint. Here's a pic of what I'm talking about. I spent 2 weeks this summer sealing these guys only to get this once I primed:
See how you can see the outline of the panels around the baffle as though they're higher than the baffle? How do I eliminate that?
An externally hosted image should be here but it was not working when we last tested it.
See how you can see the outline of the panels around the baffle as though they're higher than the baffle? How do I eliminate that?
It's been my (and a few others) experience that even if you do get it completely smooth and flush with the baffle, it will still crack through the paint. MDF absorbs moisture, especially on it's edge. Once painted, it starts losing moisture till it reaches equalibrium.
The best way to prevent this from happening is to miter the corners. No edge grain to deal with. It's precision cutting though, it can be tricky.
Another solution is one Tony Gee used in one of his projects where he wrapped the box again in thin mdf, then chamfered the corners. The joint is then on the corner, making it hard to spot if it cracks. The following sketch shows what I mean:
The best way to prevent this from happening is to miter the corners. No edge grain to deal with. It's precision cutting though, it can be tricky.
Another solution is one Tony Gee used in one of his projects where he wrapped the box again in thin mdf, then chamfered the corners. The joint is then on the corner, making it hard to spot if it cracks. The following sketch shows what I mean:
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454Casull said:
Some, but certainly not as much as when you are constantly giving it a bath with diluted PVA glue. After the first coat of oil based primer, it's sealed.
When I want to paint MDF, I sand smooth, prime with oilbased primer, then repair any nail holes, defects. Sand and prime again. It's then ready for paint.
Save the PVA glue for craft time with the kids.
Trying to get flush edges
This is one of the frustrating parts of woodworking. Most or all electric sanders have a soft pad. Though this is good for even surfaces it is not so good for seems etc. I think what you need here is a block sanding method. Make sure you use a real good filler like Elmer. I never use a power sander for the finer touches and cleaning up joints and edges are important fine touches. If this is not done with care it does not matter what you use for the finish because it will always look like cra?. A sanding block with medium grade sandpaper then fine grit light handed. (not too much pressure) Good luck.
http://elmers.com/homerepair/homerepair_products.asp?cat=2
This is one of the frustrating parts of woodworking. Most or all electric sanders have a soft pad. Though this is good for even surfaces it is not so good for seems etc. I think what you need here is a block sanding method. Make sure you use a real good filler like Elmer. I never use a power sander for the finer touches and cleaning up joints and edges are important fine touches. If this is not done with care it does not matter what you use for the finish because it will always look like cra?. A sanding block with medium grade sandpaper then fine grit light handed. (not too much pressure) Good luck.
http://elmers.com/homerepair/homerepair_products.asp?cat=2
Been there and done that. I did several coats of sealer then sanding, then primer inside and out. Sanded to perfection, and primed again, sanded again. Every time the solvent hit the MDF the edges grew more than the panel and the line appeared. Finally I just painted it with several coats of finish color and let it cure a week. Then I sanded it again, then painted a finish coat.
It looked perfect. For about 6 months. Then the baffle lines showed up through the beautiful paint.
I'm pretty sure you have to use 45 degree edges on all panel glue joints to keep this from happening. I had no luck with joints like you show. I need to build a center channel and I have some new Festool machines, so I'll try the 45 degree joints on it.
If I still have problems, then I'll coat it with fiberglass resin and do a LOT of finish sanding.
It looked perfect. For about 6 months. Then the baffle lines showed up through the beautiful paint.
I'm pretty sure you have to use 45 degree edges on all panel glue joints to keep this from happening. I had no luck with joints like you show. I need to build a center channel and I have some new Festool machines, so I'll try the 45 degree joints on it.
If I still have problems, then I'll coat it with fiberglass resin and do a LOT of finish sanding.
Here, don't take my word for it. A quote from http://www.taunton.com/finewoodworking/SkillsAndTechniques/SkillsAndTechniquesArticle.aspx?id=26508
"Always use a solvent-based primer
Solvent-based primers (oil-, alcohol-, or lacquer-based) are a must. Never use a water-based product for the initial finish coat. The wood fibers will swell too much when they absorb the water, and you'll get what is, in effect, raised grain on the surface that will not sand out. After the surface has been sealed with something ese, though, a water-based paint will not affect the MDF adversely."
"Always use a solvent-based primer
Solvent-based primers (oil-, alcohol-, or lacquer-based) are a must. Never use a water-based product for the initial finish coat. The wood fibers will swell too much when they absorb the water, and you'll get what is, in effect, raised grain on the surface that will not sand out. After the surface has been sealed with something ese, though, a water-based paint will not affect the MDF adversely."
Miters will do the trick.
So will a proper sealer after the mdf has dried thoroughly.
Polyester fiberglass resin won't do it. It is not an effective moisture vapor barrier.
Alkyd enamels, epoxy, aluminized polyurehtane all are effective at reducing moisture vapor transmission into the mdf.
Some people don't experience any problems and you know what, it won't matter too much what they use as a sealer because they aren't having a moisture problem.
Those that are having a problem need to use an effective solution.
So will a proper sealer after the mdf has dried thoroughly.
Polyester fiberglass resin won't do it. It is not an effective moisture vapor barrier.
Alkyd enamels, epoxy, aluminized polyurehtane all are effective at reducing moisture vapor transmission into the mdf.
Some people don't experience any problems and you know what, it won't matter too much what they use as a sealer because they aren't having a moisture problem.
Those that are having a problem need to use an effective solution.
Dane said:
Here's the problem with no solution other than using veneer or mitering the end pieces.
It has nothing to do with MDF per se. The problem would be the same if one were using solid wood, in fact maybe even greater...
All wood and wood by-products expand and contract due to temperature and humidity - the stuff absorbs humidity which causes it to expand and then during dry periods will dry up and contract ever so slightly. Now with horizontal and vertical pieces that are butt-joined, they will expand by different amounts, i.e., a 3/4 in. piece will not expand as much as a 12 in. piece (or is it the other way around??). In other words the wood or MDF is always working, so try as you may the joins will sooner or later show up.
One way to hide (?) the problem is to exagerate it and make it part of an overall design plan. Carve/route a small channel at the joins and paint it a different color or not... You see this a lot in all types of furniture - it's not a design effect but rather a cover up!!!
One poster a while back suggested to paint the surfaces, both interior and exterior with wood hardener. I haven't tried it yet so I couldn't vouch for it but eventually I will try it.
By the way there is no problem in using a water based primer or paint on MDF. Two coats of primer with a good sanding is all you need. Also, a watered down glue wash is an excellent primer/sealer - it is used to seal end grain of natural wood so that stain on the end grain will match the stain applied to the wood proper.
Hope this helps,
Fred P
It has nothing to do with MDF per se. The problem would be the same if one were using solid wood, in fact maybe even greater...
All wood and wood by-products expand and contract due to temperature and humidity - the stuff absorbs humidity which causes it to expand and then during dry periods will dry up and contract ever so slightly. Now with horizontal and vertical pieces that are butt-joined, they will expand by different amounts, i.e., a 3/4 in. piece will not expand as much as a 12 in. piece (or is it the other way around??). In other words the wood or MDF is always working, so try as you may the joins will sooner or later show up.
One way to hide (?) the problem is to exagerate it and make it part of an overall design plan. Carve/route a small channel at the joins and paint it a different color or not... You see this a lot in all types of furniture - it's not a design effect but rather a cover up!!!
One poster a while back suggested to paint the surfaces, both interior and exterior with wood hardener. I haven't tried it yet so I couldn't vouch for it but eventually I will try it.
By the way there is no problem in using a water based primer or paint on MDF. Two coats of primer with a good sanding is all you need. Also, a watered down glue wash is an excellent primer/sealer - it is used to seal end grain of natural wood so that stain on the end grain will match the stain applied to the wood proper.
Hope this helps,
Fred P
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