how to hide the butt edge on MDF?

[gychang]

I am building a speaker, and plan to finish with latex paint and polyurethane.

I have butt joints on the 3/4" MDF and need to hide the rough edges etc. Some have recommended "bondo" from auto parts store but more likely very difficult to sand. I have tried a joint compound but I wonder whether this will come off.

Any suggestions to which product? where ?(Homedepot or auto parts) that will help seal and hide the imperfections...

thanks,

gychang

[Tom Slick]

bondo is actually very easy to sand. you have to sand it about 20 mins after it sets up.

[downtheroad]

If you have the ability to spray, Enduro White Undercoat is excellent ....fast build and easy to sand.
See it here:

http://www.generalfinishes.com/industrial/industrial.htm

I also live in Orange County. I know you can get Enduro products at Austin Hardwoods in Santa Ana ..... 714-953-4000

[cabinetman]

If you have voids, such as holes, or crevaces, bondo will fill them. The mixing ratios of hardner to the bondo will give the "set up" time. Try to mix only as much as you need. For just "sealing" the edge to be coated with latex (water based acrylic paint), I like a product called "Gripper", a water based primer at HD. It will be difficult to completely make the butt seam between the two pieces disappear. You may also try a good spackle, and make sure its completely dry before sanding.

[gychang]

If you have the ability to spray, Enduro White Undercoat is excellent ....fast build and easy to sand.
See it here:

http://www.generalfinishes.com/industrial/industrial.htm

I also live in Orange County. I know you can get Enduro products at Austin Hardwoods in Santa Ana ..... 714-953-4000

I don't own a spray or compressor...

gychang

[gychang]

I like a product called "Gripper", a water based primer at HD. It will be difficult to completely make the butt seam between the two pieces disappear. You may also try a good spackle, and make sure its completely dry before sanding.

I have used Kilz primer, wonder if "gripper" is any better...

gychang

[cabinetman]

IMO, the water based "Gripper" is better than the water based "Kilz", and dries faster than oil based.

[Richard]

I am building a speaker, and plan to finish with latex paint and polyurethane.

I have butt joints on the 3/4" MDF and need to hide the rough edges etc. Some have recommended "bondo" from auto parts store but more likely very difficult to sand. I have tried a joint compound but I wonder whether this will come off.

Any suggestions to which product? where ?(Homedepot or auto parts) that will help seal and hide the imperfections...

thanks,

gychang

Rough edges? I have built many MDF speaker cabinets and never have rough edges. Are you using a edge trimming bit on your router? What are your construction techniques?

[siliconbauhaus]

It's a bit late now but on your next speaker build you could rabbet the edges so you dont have as much of the edge exposed.

[Tom Miller]

Are you up for trying a slightly different construction technique? Here's one that I picked up from Ray (in the UK): Use 3/4" x 3/4" stock at all mdf intersections. The 3/4" stock can be butt joined to the mdf by any technique, including biscuits, dowels, just glue, whatever.

Now, since you have hardwood at all the edges, you can profile the edges quite easily to suit your tastes.

Regards,
Tom

[cabinetman]

For a rabbeting technique that I use for hardwood plywoods to do a rabbet 3/4" x 11/16". This leave a 1/16" wing on the rabbeted piece to overlap the raw edge of the corresponding 3/4". You still wind up with a 90 deg glue joint. The actual dimension would be slightly more than 3/4" so there would be a small edge to smooth back to the 3/4".

[crokett]

Drywall compound. It even works on profiled edges. It will fill the edge. Then you can sand it, prime and paint it. On profiled edges put it on and get it as smooth as you can. Then take the router bit that made the profile and scrape it along the edge to reprofile.

[Stormbringer]

The other suggestions should help you out.

Just curious. Why not just miter the edges? No "rough" edges to cover. This was my concern when building my fireplace mantel - that the gap of a butt joint may show through over time due to settling, shrinking, change in temps, etc... especially being painted white.

Good luck,
Greg

Click to enlarge to see the edges

[gychang]

The other suggestions should help you out.

Just curious. Why not just miter the edges?

Click to enlarge to see the edges

Indeed an elegant solution, am not sure whether my skills with circular saw will be this good, but will try it with a straight edge next time.

gychang

[cabinetman]

Indeed an elegant solution, am not sure whether my skills with circular saw will be this good, but will try it with a straight edge next time.

gychang

In the beginning (sounds like a biblical story) when there was no money and I was getting started and heard the word Unisaw (which I thought was a prehistoric animal of some sorts), my saw was a circular saw. I had that thing mounted underneath a piece of plywood for a long time.

I'm not suggesting you do that, BUT, where there's a will there's a way.