I want to surprise my friend with a portrait painting of his image that will be glowing in the dark but don't know what paint to use. Any help please?
There are at least three approaches, but it will be difficult to make a "fine art" portrait, especially that looks like one in the dark. All glow-in-the-dark paints are limited to a small number of colors that tend to be primary or secondary "neon" colors. I don't know how well the colors blend to create other colors, but color inkjet and laser printers create most of the spectrum by placing small dots of primary colors next to each other, so that might be a fallback approach.
There are two general kinds of glow in the dark materials (aside from what's in glow sticks that glow for a short period after mixing, and materials that rely on radioactive material).
Phosphorescent materials can be "charged" with light energy, then they slowly release the energy as light which is visible in the dark. The light they release is not very bright, and isn't visible for long. The kind of light used to charge it makes a big difference. Direct sunlight or a full-spectrum light (like a grow lamp), give the best results. An incandescent light will give much poorer results.
The color of the light released isn't the color that the material appears in normal light, so colored phosphorescent paint is typically mixed with a colored pigment so that the color is similar in normal light.
Phosphorescent paint isn't designed for fine art, it's typically used for purposes like painting large shapes for Halloween decorations. It would be hard to create a portrait that you could actually see in detail in the dark. The paint tends to be pretty thin and requires a lot of coats to build up enough material for reasonable brightness and uniformity. With pigmented colors, a lot of the phosphorescent material gets blocked by the pigment, so it glows less brightly after it dries. You can find a range of colors like this Amazon link (no personal experience with this product, it's just an example).
Fluorescent materials are excited by ultraviolet (black) light, and give off very bright, visible light, often enough to see the effect even in room light that isn't too bright. In a dark room, the picture will glow. You just need to illuminate the portrait with an ultraviolet bulb (and it will glow as long as the UV bulb is on). You can find color assortments like this Amazon link (again, no personal experience with the product).
So two of the approaches are to use either phosphorescent paint, or fluorescent paint plus a UV bulb, and create the colors as best you can from what's available. Of the two, the latter is likely to give you the best result.
The third approach would be to create a monochrome portrait with non-colored phosphorescent paint. You create shading by how thickly you apply the paint (number of layers; the thicker the paint the brighter it will be). This would be analogous to packing tape art, although light and dark would be the reverse. I'm not sure how easy it would be to discern the detail in normal light.
I'll throw in one more idea, although I don't know if it would work. You can buy or make non-colored phosphorescent paint (just a clear vehicle plus strontium aluminate powder). You might be able to create a normal portrait, then coat it with a thin layer of this paint. It will add a haze on top that will mask fine detail. But it might work to illuminate the portrait in the dark. It wouldn't be very bright, so the portrait would need exaggerated contrast.
You can buy phosphorescent vinyl sheets (example link). I've used this material under blacklights (UV lamps) to glow long term; the UV charges the phosphor as it discharges. These days you could use UV LEDs.
You'd then need to paint on top of a (typically) slightly textured vinyl. I haven't found a source of white material, so it would be the typical ghostly green of phosphorescent signs, and you'd be best with monochrome art. Front-lit, even translucent red paint won't allow a huge amount of charging, but back-illumination with a blacklight might work.
Phosphorescent white paints do exist. You could perhaps use them under translucent paints (e.g. glass paint) with UV illumination, but I haven't tried it.
Finally an artistic friend used common highlighter pens to draw on black fabric. Even as she drew it the design was invisible, so she must have had a good memory. When taken under the blacklights the picture appeared. The problem again is the lack of colours: the blue and purple shades are a lot less intense even under blacklights than yellow/orange/pink, and there isn't a true red. Invisible UV paint (as mentioned in Fixer's answer) would give you a little more control and more colours; you'd probably need to work under UV light.