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.