How to keep the seams from showing up on MDF?

Joseph Shaul

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206
Location
Madison, WI
I have a set of unfinished MDF speaker boxes I'd like to finish, and was wondering if anyone has any tips as to how to prevent the seams from showing through the paint. The boxes were made some time ago and left unfinished, and the expansion of various relative pieces of MDF is visible; while the construction is solid, I'd like to know if there's anything I can do beyond sanding it flat and hoping for the best.

Incidentally, can anyone tell me how to figure out if my black shellac is past-due?
 
to me if you have seems that you dont want to see you need to cover them up with a solid piece rather than a two piece cover.. and as for the shellac try a sample out on so scrap and see if it drys.. if it does you should be ok to go
 

The box has a roundover on both the top and sides - otherwise, I'd have taken the easy option. That said, I have seen seams poke through veneer before - maybe a laminate of 1/64" plywood layers before the veneer would prevent this?

Just sand it to a finish surface (320 or better).

Black shellac???

I have no idea exactly what the stuff is, but it was free and it's supposed to work very well. I got it from another DIYer who was moving to an apartment and clearing out his stuff.
 
I have no idea exactly what the stuff is, but it was free and it's supposed to work very well. I got it from another DIYer who was moving to an apartment and clearing out his stuff.

That sounds... kinda' freaky. Are you sure it's really shellac at all? Does it smell like denatured alcohol, burn freely with a blue flame? Real shellac should. If it's unclear what it is, then it's unclear what it'll do, how it'll act.
 
Any chance it's black lacquer? :dunno: I still think I'd seriously test it out before putting it on something I cared about. Might be cheaper and more expedient to just go buy a can of black high gloss enamel from the paint store.

Oh, and ditto on the Bondo suggestion. :thumb: Best stuff since duct tape.
 
Any chance it's black lacquer? :dunno: I still think I'd seriously test it out before putting it on something I cared about. Might be cheaper and more expedient to just go buy a can of black high gloss enamel from the paint store.

Oh, and ditto on the Bondo suggestion. :thumb: Best stuff since duct tape.

I'll try the Bondo.

What black lacquers do folks here recommend for a "piano finish"?
 
.....What black lacquers do folks here recommend for a "piano finish"?

Any quality black lacquer will work. Creating a piano finish is a process of building clear layers, sanding , more building , more sanding and buffing.
Sometimes I may have to build 5 to 7 heavy coats of pre-cat lacquer with lots of sanding in between coats until it is ready for buffing. It is labor intensive, sandpaper intensive and time consuming, but not difficult as long as you dont cheat along the way. Most people are very satisfied with a good quality finish straight from the gun. I usually charge about $350 extra for a mirror finish on a table top. The terms mirror finish and piano finish are synonamous. "Piano finish" goes back to the days when only piano's had that mirror finish.
If you are really interested, you might want to open a different thread and see what others have to offer.
 
I'd stick with Larry's advice in addition to the others. As for the finish, Larry's advice would apply, do a test piece and it will test not only the joint, but the finish itself to see if it is any good.
 
I'd stick with Larry's advice in addition to the others. As for the finish, Larry's advice would apply, do a test piece and it will test not only the joint, but the finish itself to see if it is any good.

I'm not sure how I'd avoid showing the seams on an MDF box. The rounded corners prevent me from using any sort of veneer. Maybe if I built a box with mitered corners?
 
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