MDF shrinking?

[wingnut]

> and the last thing we want to see is a crack
> in a **** joint.

I prefer joint cracks over **** cracks, though -- unless you're talking my wife;D

[ericanderson]

> let the MDF acclimate to the temperature and
> humidity in your workshop for a couple of
> weeks before cutting, rather than cut it the
> minute you get it home (unless where you got
> it, your car and your workshop have pretty
> much the same temperature and humidity).
> Paul

I put together an MDF sub using **** joints and 3/4 MDF glued with yellow wood glue (PVA). I am in the process of sanding and finishing it (a week or so after glueup). I am using automotive grey primer/surfacer. I detect a slight hint in the **** joint with the primer sanded to 320, but I'll let you know what a black finish with a sheen of about 15 (nearly dead flat) looks like when it is done. I guess I am surprised to see anything a week after glueup. If the joints show, then there must be a difference in the density of the MDF in thickness VS length/width.

[bobbarkto]

Sanded primer has a slight gloss, about 45 or so. When you go flat I bet the effect is very nearly invisible.

I don't think it's a problem of variations in density.
The end of one panel is rigidly attached across the thickness and so it will be restrained at that point. The panel will shrink evenly away from the point of restraint. The effect will be measurable and more or less equal on both surfaces.
Maybe a pic...

> I put together an MDF sub using **** joints
> and 3/4 MDF glued with yellow wood glue
> (PVA). I am in the process of sanding and
> finishing it (a week or so after glueup). I
> am using automotive grey primer/surfacer. I
> detect a slight hint in the **** joint with
> the primer sanded to 320, but I'll let you
> know what a black finish with a sheen of
> about 15 (nearly dead flat) looks like when
> it is done. I guess I am surprised to see
> anything a week after glueup. If the joints
> show, then there must be a difference in the
> density of the MDF in thickness VS
> length/width.