For those big, hot, heavy guns! Mimics the colour changes you see happens to steel etc under heat.

Basecoat

1. Paint metal with Leadbelcher
2. Wash with a mix of Nuln Oil , Nuln Oil Gloss and Agrax Earthshade . Ratio is probably 3:2:1. I use Agrax Earthshade as this is a Chaos Knight weapon, I'd probably go 1:1 of both Nuln Oil shades otherwise.
3. Drybrush Leadbelcher , then again with Stormhost Silver . I ran this sidewise along the barrels as well the usual fast drybrush strokes.

Muzzle Effect

Firstly look at welding or heated steel reference images online. You will need to plan where you wish the heat affect to go, and how strong. You need to ensure you get your paint ready on the palette, and use an appropriate-sized brush. I used a good Size 2 because of the gun size. Paints needed are: Drakenhof Nightshade Druchii Violet Carroburg Crimson Casandora Yellow Lahmian Medium , Guilliman Blue and Abaddon Black .

1. Apply the Drakenhof Nightshade . Move quickly and keep it wet. This is the hottest the metal gets so plan where you intend this goes.
2. Next is Druchii Violet . Not as wide as the blue, and just stroke the edges together. Work across the edges with the side of the brush. Transitions might not be massively smooth at this stage but should be better than a line.
3. The Carroburg Crimson is just to give the mix from purple to yellow a more natural transition and I ran across the edge of the Druchii Violet , creating a Carroburg Crimson layer maybe half the width of the previous.
4. The Casandora Yellow stage is quite wide, and you will want to pull the brush up from where you want it to end into the Carroburg Crimson layer. This drags the wash up, leaving it less intense further away from the barrel end.
5. I used Lahmian Medium to quickly blend the end of the Casandora Yellow stage to blend into the metal.
6. Repeat the stages where the wash isn't as strong as you would like. I did two layers of all the steps aside from the Lahmian Medium stage as the yellow to metal transition was already smooth. I used Guilliman Blue at this stage all over the :Drakenhof and partially over the Druchii Violet to add intensity to the colour.

Final Touches

1. Mix water into Abaddon Black to make a wash. Run this in the gun holes, exhausts etc. This is more intense than Nuln Oil but a lot more matte than Black Templar .
2. Apply a very light drybrush of Stormhost Silver