how to fill/paint letters that i have engraved in wood ?

[martin54]

I think standard procedures for painting text like this is to use a syringe, needle and ink to fill the letter 'pockets'. Mind you this is on metal, so in your case I would paint the piece with waterproof paint to stop the ink sinking into the MDF and when everything is dry, use the ink method.

Regards,
dsc.

Don't know about infilling letters with ink but it was certainly done infilling the letters with paint, normally something like "One Shot" which is a traditional signwritting paint. Once the piece had dried it was then buffed & polished to ensure that it was flat & any high spots in the paint were removed.
MDF is porous so I would think it would need to be sealed in some way before any painting.