How Do I Begin To Sand MDF?

[Bing Fung]

Ok, here I am after two applications of wood filler to all my screw holes, ready to sand the the filler and MDF joints.

What sequence of sand paper should I use. ie begining grit, finish grit?

I have perpendicular joints (ie front baffle, top deck) where the seam is about 1/32 higher on the top deck, will I run into problems with this? Do I just sand the top deck edge down to the front baffle edge?

Thomas, I found some Black Oak, gel stain, should I use that? From the samples I saw it looks to be the finish I'm looking for, I believe it's a stain and varnish all in one.




Bing

[ThomasW]

Let's see I forgot to tell you about using "Bondo"(lightweight body filler)to fill the holes. Oops, well just be careful sanding, frequently wood filler doesn't stick too well to the tops of the wood screws.

Anyway the best way to flush trim a butt joint, is with a router and a flush trim (bearing under)bit. It the top piece is overlapping the side, the bit just zips it off perfectally. It's almost impossible to use a sander to get a square edge on a butt joint.

If the top piece is too short compared to the side, build it up with Bondo, and again use a flush trim router bit.

For sanding use 80 grit to rough sand, then 120 or so to smooth everything out. Since you're going to veneer, you don't need or want the MDF mirror smooth. Just completely flat, with good square corners.

The gel strain will work fine. I'd still give the whole thing a couple coats of spray clear poly for max protection. Doing this allows minor scratches to be sanded out, without messing up the stained surface underneath




theAudioWorx
Klone-Audio

[Bing Fung]

Damn, I was thinking the same thing as I was appling the filler "I hope this stuff sticks to the screws..."



My thought was to maybe glue on a strip of veneer on the end of the baffle to bring it to level with the top. Then I would sand it down to the top deck level. I have this thought that seeing as the difference is less than 1/32" the filler may tend to flake off...




Bing

[ThomasW]

Try popping out the filler from one screw. If it is stuck reasonably well, no worries.

Anything that will fill the void is ok. Bondo although a pain (due to the toxic fumes) is easy to "feather" to a fine edge. I use it for all things related to "fixing" MDF, since it can be worked in a very short time after it sets up.

Don't use wood filler to "fix" the edge. It won't hold.




theAudioWorx
Klone-Audio

[Bing Fung]

Ok, I pried one out and it seems relativly stuck.

Well Thomas, i took a sander to the edge, seeing as I don't have a router handy, I can see what you mean with this square edge cut. This sander sucks and mu edge is kinda like a roller coaster now , im hoping it isn't as bad as it feels to my hand.




Bing

[ThomasW]

If this is a front edge it will most likely be covered by the grill

If it's a side then you better get it square before putting on the veneer. Any variation you can "feel", will be "magnified" when you put on the veneer




theAudioWorx
Klone-Audio

[Bing Fung]

It's the front edge.




Bing

[ThomasW]

Take a scrap piece of 2X4 and staple a full length piece of sandpaper to it. Use this to even out the edge. Then make sure to get good pressure on the edge, when gluing down the veneer




theAudioWorx
Klone-Audio

[ThomasW]

oops posted to the wrong thread




theAudioWorx
Klone-Audio

[outhok]

Wrap a piece of scrap wood with wax paper or plastic wrap then tack it to the outside of the box. Place the top edge at the same height as the main surface. Fill the void with bondo. Sand flush with 80 grit after about 30 minutes. Repeat if necessary. Remove the board and finish sand.