I have some loudspeakers with the front panel made of MDF, on which a matte plastic foil was attached.

After 15 years the glue is ineffective and the plastic shrunk, so I would like to remove it completely and I would like to apply some primer and black matte paint, both acrylic/water based (I understood it's better suited to my case than oil based).

I will sand the front panel everywhere, but how can I ensure, without applying primer, that I actually removed all the glue residues?

Should I use the colour of the surface, should I use a damp cloth, or is the primer the final test and I should simply sand again the parts where it's not sticking?

I don't have much experience with wood working, in the past I built something but I used new wood that required no treatment before painting.

Edit: sorry, I initially wrongly wrote particle board instead of MDF.

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    you might want to test on an inconspicuous area. I don't think you'll need to remove all the glue, since the primer will help the paint adhere. You can probably just scrape/sand until you're comfortable with the surface texture/smoothness, then prime and paint. – aaron Feb 6 '18 at 13:47
  • Definitely don't wipe MDF with water. Water will ruin MDF with a quickness. – Charlie Kilian Feb 6 '18 at 15:34
  • @CharlieKilian do you mean that I also shouldn't use water-based primer and paint? – FarO Feb 6 '18 at 15:53
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    No, you can use water-based paint and primer. You just can't put water itself onto MDF. Mostly it's about drips or spills -- unsealed MDF will start to delaminate when it gets wet. But it's sensitive enough that I wouldn't want to wipe it with a cloth wet by water to check for glue spots. Honestly, I don't think you should need to do that with MDF. That's more of a trick for wood or veneered plywood. Anything that has actual wood grain. It won't do anything for MDF, and MDF doesn't react well to water, so I wouldn't put water on MDF on purpose. – Charlie Kilian Feb 6 '18 at 17:00
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    It's entirely possible I'm being overly cautious here. But still, I don't think this trick would work well for MDF anyway, and in case you didn't know how MDF reacts to water, I thought it was worth mentioning it. – Charlie Kilian Feb 6 '18 at 17:02

If you are applying a pigmented finish like paint then it really doesn't matter, this is usually only a concern with clear finishes or stains.

But if you really want to see it, wiping down with a solvent like mineral spirits, paint thinner, denatured alcohol or water (will raise the grain) should show you any spots you missed.

  • There is no grain to raise; the front panel is MDF. He shouldn't wipe it with water, the MDF will delaminate and bubble up. – Charlie Kilian Feb 6 '18 at 15:36
  • Thanks @CharlieKilian I am aware of this but as a general answer.... – Steven Feb 6 '18 at 16:17

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