Trying to get flush edges for paint prep

[lemans23]

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!

[MJL21193]

Here's a better tool:

[neutron7]

what kind of wood is it? maybe you need some sanding filler, it fills gaps and bits of the grain which would soak up too much finish.

[bobhayes]

A proven method to combat your problem is to actually 'groove' the line deeper and wider, and then to use a filler, prior to sanding smooth. A router is ideal, but you can use a chisel.
Make sure your panels are well bonded together in order to prevent any movement in the future.

[PoorboyMike]

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!

[Kinnja]

Originally posted by PoorboyMike

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.

I haven't built any speaker cabinets yet, but I have seen trim carpenters use bondo in a similar fashion.

[lemans23]

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?

[MJL21193]

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:

[MJL21193]

Originally posted by lemans23
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.

Whoever said PVA glue was a suitable primer?? That's part of your problem right there. After sanding prime with REAL primer, not home-brew. First coat of anything that goes on MDF should NOT be water based. Use oil/solvent based primer.

[454Casull]

Originally posted by MJL21193

First coat of anything that goes on MDF should NOT be water based. Use oil/solvent based primer.

Yeah, I was going to mention that. But does MDF still rise with the non-water-based primer?