high gloss piano black speakers

[tryinhard]

Hi all, I know this question has been asked before but I'm doing something different. I'm making speakers, not your normal square box type, they are octagonal and made of mdf. I've looked at a lot of posts on a lot of sites trying to find out how to stop the joins from telegraphing through the finish.
I have used so far PVA, polyurathane and epoxy glues to no avail. I have used automotive primer, undercoat, topcoat but the joins slowly reappear.
This happens not only on the ends but also along the sides as well, any suggestions are welcome.

PS. The octagons are three different sizes approx 150,300,450 mm diameter and are mitred at 22.5 degress to join eight together with 18 mm MDF.

[vsquizz]

Have you tried biscuit jointing instead of plain mitres??


Cheers

[Gra]

Have you tried Automotive body filler, Commonly refered to as bog or bondo? this seems to work.

[Harry72]

Paint the MDF joins with fibreglass resin, be generous it will soak in. Once its set sand back flat then fill a voids with car bog. Do not apply car bog directly in the mdf joins as after a while it pulls away leaving gaps... mdf and bog have way different shrink rates when moisture is added, coating the mdf in resin practicaly waterproofs it!(coat the whole enclosure...inside and out to seal it)
It would'nt hurt to use F/Glass & resin as an adheasive, you can use that pre-pregnated fibre/bog stuff, an old car audio sub box trick from way back.

[soundman]

A friend had similar problems with a few speakers he was manufacturing in quantity from mdf.
There are some inherant stability problems that cause the movement. MDF is considered stable & consistant but It isnt completely immobile also when you you mitre you multiply the error.

Use of glues with a crisper nature or ones with fillers will give improvements. This felow went from common pva to AV180 which is crisper & gained some improvement. A hard chrisp glue like urea/formaldhyde may get you close.
Fillers won't solve your problem, tried all sorts.
The place where you will have the worst problems is not on the mitred joins but where the hex meets the flat if you are trying to finish flush.

my recomendation
redesign so problem joints are either eliminated, concieled or desguised.
use a crisp glue the glues you mention all have a plastic nature & prone to creep.
allow the box to stabilise a week or so to allow for glue shrinkage ect.
perhaps use a flush trim & glue line groove bit on some seams (that puts a decorative groove on the glue line).

We have seen some MDF speaker boxes glued together with some sort of filler like product a bit like cornice cement, never did figure out what it was but it left a good joint & sanded real clean.
cheers