Zinc Phosphate search

[Ron308]

Hello Rob!
I mix Ospho with water in a 30-1 ratio (30 parts water to 1 part Ospho) and run it up to a temperature that's just before boiling. 175-180f should be about right, if you have a meat thermometer handy. Once you start to see some steam coming off, you'll know you're close.
I use a stainless tank of the appropriate size and a propane heater for my heat source. You should always do it outside because the fumes are dangerous.
After blasting the metal, I degrease it with brake cleaner (old formula, not the "green" stuff) and blow it dry (compressed air). But it has to be "clean" air. Like if you were running a paint gun, with a drying filter. You can't have any oil contamination or the park won't take where the oil (including fingerprints) are or you can let the brake cleaner evaporate off the parts naturally if you don't have clean air.
Then I suspend the parts with wire into the solution (or for small stuff, just toss them in) and let them soak until they achieve the darkness of color I am after. You have to be quick in the time between finishing the prep and submerging the parts into the solution so they don't get rusty.
The parts should "bubble" as the solution does it's thing. I will continuously agitate the parts to ensure a even coverage. For thin or small parts, it doesn't take long, maybe 5-10 minutes. Thick parts, like a bolt, might take 10 more minutes longer. And not all the parts will end up with the same shade. Thicker or harder parts will usually end up darker than thin and small parts.
I remove the parts from the solution and if I deem them done, I transfer them directly to a hot water bath and agitate them to get all the solution out of the nooks and crannies. Once I remove them from the water, I dry them and quickly oil them to set the park and prevent flash rusting.
Keep an eye on your solution level. If it gets low (as it will as the water evaporates) add more water to compensate.
The vapors are harmful so wear appropriate PPE. A dust mask isn't good enough, you should use some kind of respirator. And because Ospho is an acid, make sure you don't get any on your skin or in your eyes. Wear glasses and gloves.
When you're done, let the solution cool and you can bottle and reuse it.
Sludge and debris will collect at the bottom of the container as the solution cools, you can salvage all the solution to reuse but don't keep or reuse the dregs.
It may sound tedious, but I've been doing it for years and I find it to be a lot of fun.