If you’ve already jumped headfirst into the world of powder coating you’ve surely encountered some stubborn parts to coat. Cast aluminum parts can be one of the most fickle to powder coat. . On modern automobiles the majority of the bolt on parts (and even some of the major engine parts!) are cast aluminum. These cast aluminum parts can really dress up your engine bay once powder coated, but they can also be VERY difficult to get 100% clean when you try to powder coat them. The most difficult items to get a smooth finish when powder coating are the ones that have been exposed to oil and grease their entire lives. Cast aluminum parts are very porous and they tend to “soak up” the oil through the minuscule pores on the surface. This means wiping the surface quickly with a parts cleaner isn’t enough. If you take a few additional steps to prepare your cast aluminum part you can have a better chance of avoiding fisheyes, bubbles, etc. Below are the basic steps to take to get your cast aluminum parts to come out perfect every time!
1. Remove loose/heavy dirt and corrosion- The reasons here are obvious, powder coating is only as durable as the surface it has been sprayed over! Any loose corrosion or grease can be wiped off and so can the powder coating you put over it. I like to start by taking a wire brush and going over the entire part to remove and the heavier corrosion and dirt. I then spray the part with a heavy parts cleaner like Eastwood Chassis Kleen to wash off the residual grease and dirt. Another reason to take these steps first is to save your time with the media blaster AND save the media in a blast cabinet. If you just throw a super dirty part into the cabinet the grease and heavy dirt can eventually mix with the media and clog the blasting nozzle, requiring a media change in the cabinet.
2. Media Blast Parts- With the parts relatively clean to the touch I then move on to media blasting the parts. The type of blaster you use is dependant on a few things; what size compressor you have, how often you media blast, and the size of your parts. If you’re regularly blasting small to medium size parts a blast cabinet will be the most cost effective, while our small blast kit does not require a large compressor and doesn’t take up a lot of space in the workshop. If you plan to blast larger parts often a standup pressure blaster is a good investment as well. A good all-around media to use on cast parts is 60 or 90 grit Aluminum Oxide. The result will be a light silver finish that is VERY susceptible to dirty fingers, so make sure you wear rubber gloves when handling the parts from here! I also like to spray the part down with Afterblast to etch the parts and clean off any blasting residue.
3. Now you’re thinking your parts are plenty clean and ready for powder right? NOPE! There’s grease and oily residue hiding deep in the pores of cast aluminum parts that can only be “burnt” out by baking or a torch. The easiest way to get rid of the hidden contaminants is to pre-bake your part in the oven at a slightly higher temperature than your powder cures at for longer than your powder cure time. This will burn off the residue and it’s the best way to out gas a part so that you won’t have fish eyes or small bubbles on your cast parts after curing the powder. I’ve also used a small torch and run it over the part to burn off the excess residue, but you must be careful to monitor the heat of the part before applying the powder.
4. Applying Powder- Larger cast aluminum parts like transmission cases, wheels, cylinder heads, engine blocks, etc. can be difficult to get the current to pass through when powder coating a cold part and poor powder adhesion can be the result. You could make a “ground tree” and attach grounds clamps at numerous points on the part and link the grounds with wire to the main gun ground clamp, but if you need to spin the part when applying the powder you will have a mess of wires. I prefer instead to apply the powder immediately after I have pre-baked or outgassed the part in the oven. Hot-Flocking the powder onto the part allows the powder to melt almost instantly when applying it to the surface and it also helps powder get into tight areas where Faraday Cage effect would not allow powder to sit. When hot flocking the powder you want to make sure you move quick and that you closely monitor the heat of the part so you don’t have powder that is half and uncured next to each other. You also need to resist the temptation of going over an area too many times as build thickness is hard to monitor when hot flocking and you can actually RUN the powder (ask me how I know!) and when it flows out it will run and create large, rock hard bubbles or runs in the powder that have to be mechanically removed.
The great thing about powder coating cast aluminum parts (especially poor quality castings) is that the powder fills and builds nicely and will make a cast aluminum part look worlds better because it will fill minor pits and casting marks in a part leaving a nice deep, smooth finish. Once you’ve got this process down it becomes second nature, but it IS necessary to get the best results on cast aluminum parts! Cleanliness is next to godliness in the powder coating world for sure!