After seeing Seimpre La Luna's painting, I decided it was time to give enamel paint a try, and I am so much happier with the result.
I have perfected the technique, I think. I use a 5mm x 15mm piece of thin flexible plastic, cut from the clear plastic that you find in packaging. I use this piece of plastic a a mini-squeegee. Just like silk-screening, I use the squeegee to wipe off excess paint.
Here's the step-by-step guide:
1. Use a good enamel paint. I'm using Tamiya Gloss White (X-2)
2. Apply paint to the engraving so it fills up to the backing paper. Do one word or about five letters at a time.
3. Remove excess paint using the squeegee. Use firm pressure, but avoid scraping off isolated pieces of mask, like inside "A" and "P". Scrape off in two directions.
4. Leave to dry at least one hour. The paint should have lowered into the engraving.
5. Do a second coat (repeat steps 2 and 3).
6. Leave to dry overnight! (I guess 6-8 hours would be enough).
7. Peel off mask. Paint should not be stringy, and the mask should not pull paint out of the engraving.
It only takes two coats of enamel paint to give a solid fill, and still be lower than the top surface of the acrylic. Wiping off the excess using the squeegee means you can fill the engraving to the top... my previous method of using a paper towel to remove excess paint would soak up the paint in the engraving and require more coats.
POIDH? Here you go. Note the triangle inside the "A" of "SAMMICH" came off during the squeegee phase... this was fixed with a craft knife, and I could touch up the black line to the right... but I won't bother. It looks better in real life - the camera picks up every spot of dust and paint flecks.