I wanted to make the best of the redesigned Poison Spyder emblem embossed on their diff covers. You could use this technique for other raw metal diff covers too.
I used POR15 glossy black for the diff cover base coat and went with the spray can for this job.
I also bought their cleaner/degreaser and metal prep solutions. The cleaner smelled suspiciously similar to TSP and the metal prep smelled a lot like naval jelly. Perhaps those could be used instead of the POR15 system, but at least I have manufacturer's assurance of compatibility.
For the color accent I wanted to match (as much as is reasonable for a differential cover) my Sunburst Orange Pearl Jeep. I went to the local Sherwin Williams Automotive and asked them if they could pull up the color code. They were able to do that and mix up 2 oz of "touch-up" type of paint that didn't have to be mixed with a hardener or sprayed on. It was about $20 but you get a lot more than the touch up tubes from the stealership.
You'll need a couple of small paintbrushes. I used a medium sized Testors model paint brush.
It helps to have a big brush for cleaning the raw diff cover. Getting into the bolt recesses is tough.
Gloves- the prep chemicals have alarming warnings about organ failure and they do a number on your skin at the least.
First thing you have to do is wash down the diff cover to get the grease and such removed. The POR15 cleaner gets 50% diluted with hot water. I cleaned mine in a bathtub. I also resorted to a big brush to really get in the bolt hole recesses. Lots of smut hides in there and you'll see it on the paper towels during drying if you don't use the elbow grease. Next comes the rinse, which will lead to immediate development of fine rust. That will be removed next during metal prep.
Because the metal prep product results in a chemical reaction, I did this part on top of a cardboard box rather than the tub. A foam brush worked really well. Starting with the inside (which in hindsight may not have been necessary) you just apply the liquid to the diff cover and "keep it wet" with the metal prep solution for 10-20 minutes. I did about 10 then rinsed it well with hot water.
It starts to dry quickly after a hot rinse and you'll see a white or grey chalkiness similar to what you see when you use naval jelly on rusty metal. I've always found that paint sticks to this very well. The instructions say you need to get the thing "bone dry" before applying the POR15. I used a propane torch to warm it up and dry it off for painting in the cold garage.
Once it's dry and your painting area is ready you can begin spraying on the basecoat. I wound up doing two coats. The first coat didn't look great as the paint sealed and filled the surface. You have to be careful to get the spray into the bolt recesses without building up too much paint and getting a run or sag. Just move quicky with the spray head. I only waited about an hour between coats because I had a floor heater blowing into a box containing the diff (outside in the cold).
While the black basecoat still had some drag, but not quite tack, I started with the accent color. The accent paint starts to cure quite quickly. You have to keep the brush wet and avoid going back over areas you covered already. The raised area makes it easy to paint without spilling over into the black area. Just don't use too much brush pressure. Try to drag it across the surface like Wliey Coyote running off a cliff. If you spill over you can touch it up with POR15 later. If a spot looks funky, dip your brush again to fix any touchup areas.
I achieved opacity with two coats, only about an hour apart. The first coat will look lame and it should until you build up some thickness. You also have to work in small sections, maybe the size of a quarter at a time. Don't do long strokes.
The finish is not going to be a perfect match with pearlescent paint for a number of reasons, but the color and effect are close. Metallic paints will have a better result. The idea is to get it close, then when you run this sucker into a stump or a rock, you just pull out your touchup paint and you're good to go.
After mounting you can touch up the edges near the gasket surface and around the fill plug with a q-tip soaked on POR15. You could do the plug this way after you fill the diff, but make sure you degrease it well and either way- don't paint the plug threads.