Malefic Warlock. Instead of weapons, you could absolutely use it on vehicles and other armour plates, trees, walls, roads even the skin of large creatures - sky's really the limit.
Most hobby stores like Michaels and Hobby Lobby will carry it and it's pretty cheap. From what I've seen, every brand works roughly the same. This is a short post, because really, it's simple stuff.
Wanting a bright red, I gesso'd over the black primer first to allow the red to be really strong. While only small cracks will show through, you could add a little detail at this point. Don't waste time doing a lot since most will be covered, but you could do a quick wash or highlight to add some natural detail to the undercoat. (Especially if the surface is rougher.)
This brand recommended leaving at least an hour for the medium to dry... I ended up leaving it overnight because I was tired. Read the directions with whatever brand you get. Thicker / thinner coats usually affects the size of the crackles.
Once it's time, paint your top coat on. It recommended not using watery paint and I've run into problems before using washes over crackle medium.
The paint starts to constrict very quickly, but I'd still leave a while for it to completely dry. You're left with a nice crackle pattern. You'll notice there's a little texture to this pattern, with the patches raised above the cracks some.
I ended up using some pigments to add brown and orange tints to the black, giving the dagger a bit of a rock/magma look. Do whatever you want.
- On a sword, you could paint white into the cracks and make it look like energy crackling along the blade.
- On skin, the edges of the patches could be painted to appear diseased and pealing.
- Wood and stone could be weathered.
- You could paint the base and top coat the same and with a wash / dry-brush use the texture it makes to just have a cracked surface.