I've been asked this question a million times and no doubt, will be asked about it a million more. It's just one of those things that's always going to be there in the field of powder coatings. I'm talking about outgassing,folks.
To understand the "why" this is happening....you must first understand the basic principals of metallurgy. For this excercise we will classify our main goal in the
non-ferrous family (aluminum,copper,lead,zinc,nickel,tin,aluminum alloy etc. Basically....anything that isn't magnetic). While
ferrous metals tend to be magnetic by nature and have a tight knit bond after heating (steel is a prime example).
Now....with that said, an aluminum metal is our basic focus here. Aluminum is one of those lovely "white" metals that likes to be very moldable and for it's weight, quite strong in a great many applications. Is it the best to hold a coating though? Not really. By nature (unless alloyed with another metal or super heated in a treatment process) it's very porous by nature. That is to say.... it allows or admits the passage of gas through pores. You'll notice that I didn't say "contains gas". Because basically...it does not. Whenever you have something that "outgases" such as your big-block intake manifold...the gas that escapes is nothing more than the air you have surrounding you. Basically the holes open up a little more due to the fact that heat expands metal and air pockets are more easily acceptable to become free of thier bonds as a bubble. What does this mean to you and how can it be dealt with,you ask? Well.....read on.
For this example we'll be taking a common part such as an intake engine manifold and coating it. Something we can all basically associate with,right? We'll be using the hobbyist gun and say..... your basic black poly powder to coat with. Sound fair?(7-10 PSI and 20 KV on the volatge side)
First steps first....I'll assume by now you have prepped your part accordingly and have it all ready to coat. At this point is our first "tip" to coating aluminum. If you have degreased the manifold....make sure you have all the solvent out. "Solvent popping" looks exactly the same as outgas and needs to be taken care of in the same manner.
Pre-Heat the manifold to a temperature of 250 degrees before coating to work it all out of the pores. Remove the piece and coat as you normally would any other metal....the pre-heat will not only solve the solvent dilemma, but attract the powder you are spraying to the part. All while this is happening, the pores of the metal have pretty much "gassed" themselves out by now and are willing to be filled with coating as opposed to air. Work quickly! Once aluminum reaches a point below 200 degrees, it likes to take the air back in, as it were. Put your part back in the oven and bake for the
low temperature guidelines (lower temperature and longer time as per your directions). Lower temperatures for longer mean less of a chance of any other pores that havn't opened up yet to stay that way. Higher heat = more open pores. Plain and simple. Let the manifold sit in the 250 degree oven until the
PMT has reached that point as well. Keep raising the temperature by 25 degrees and waiting a good 5-10 minutes (depending on thisckness of the metal) until the
PMT itself matches the ambient air in the oven. You'll keep doing this until the point of your lowest cure temperature has been reached. Good..... now cure as per your directions (300 degrees for basically 22-28 minutes PMT sounds about right on a poly).
So far we have blasted and prepped aluminum, pre-heated then coated. In the oven slowly up to cure temperature and now we are about to remove. If this were a convection oven I'd just say shut the heat off and crack the doors (for all you advanced hobbyists), but for all the kitchen oven users out there....removing and letting air cool at this point is just the same. The point is to get that PMT down rapidly. Enough to where the pores of that metal will want to close up quickly and take the coating with it. (side note....this is where coatings on cast aluminum done properly will have superior adhesion. A pleasant side-effect for us, the powder coaters of the world) What you
should have in the end of it all is basically a well coated piece of cast aluminum. Sure...there's tons of other methods of coating and primers involved but for you guys....this is where you start. Second coats should pretty much be treated the same way but with more comfort in knowing that you can raise the temperatures quicker if you like without fear of further outgassing. Keep in mind...if you cure a part at 300 degrees for the first coat, you should do the same for your second coat as well! Anything else and it will no doubt create a gas bubble underneath your coating and eventually failure down the road. Once the coating is cured however.... it can be treated like any other part you've done. Just as durable, just as sexy a finish
Sorry that got long winded...but like I've said in the past....some of these things need to be at times. Hope that helps somebody out there (and thanks to user "tomg552001" for the information request that sparked this post). Take care all.....Russ