How can I dye MDF?

Question

I have a bunch of sheets of 2mm-thick MDF that I'd like to dye black for use as a guitar headstock overlay. Since this would typically be laminated to the headstock and then trimmed flush, its colour should be through-and-through.

One idea I have is to put the piece in a baggy with the dye (India ink is what I have) and clamp it in a vise between other pieces so that it doesn't swell... I'm also considering whether I need to sand off the shiny finished surface of the MDF in case it's impermeable...

I'd be happy to avoid hours of messy experimentation, so I'm wondering if anyone has experience with this?

Answer 1

You will get some penetration, but much less than a millimetre. One thing that should work is to trim very nearly flush, then mask the rest of the headstock and brush or wipe dye onto the sanded face, before smoothing with fine sandpaper (as any water in the dye will cause a little swelling). If you sand off too much, apply more dye, and while sanding, keep the masking in place. If the part to which you're laminating it is already well-varnished, the varnish may be sufficient to protect the existing headstock.

If you can use an alcohol-based dye you'll get a lot less swelling, but I wouldn't rely on the varnish protecting the headstock unless I knew exactly what it was.

Demonstrating the lack of penetration: pressure-treated wood for outdoor use is designed to maximise takeup of the treatment. Cut across it and you'll see just how little of the cross-section is treated. MDF will be more absorbent, but not enough to change things dramatically, and this also means it suffers more than wood from being saturated.

Answer 2

This is not really an answer to your title question, and more of a solution to your situation, but, as fixer1234 mentions, no colouring method will evenly pervade the MDF fibres after they have been pressed while keeping the overall structure intact.

Nevertheless, coloured MDF can be bought in several places and online (e.g. 'Coloured MDF', 'Colored MDF', 'Vibracol', 'Color MDF'), and especially in black it is not hard to come by. Since some of the linked producers offer samples for free, you might even be able to get a sufficiently large black-coloured piece for free.

^{Coloured mdf, so-called Valchromat: source}

Not all of these coloured MDF sheets seem dark enough, though, so you probably have to coat them manually anyway, and, since the finished surface is indeed impermeable by liquids, you'll have to sand, ground, and coat that surface.

But since the MDF sheets you already have are only usable if you concede to painting your custom overlay after they have been attached to the headstock and trimmed flush, consider getting a piece of black veneer or ebony and simply use that.