I found sand blasting aluminum prior to powder coating is a big no-no.

[mtumbleson]

I have intended on sharing my findings for some time, but I wasn't not sure how it would be received. Keep in mind that this is extruded aluminum NOT cast. The process I use (or don't use) has been proven on several hundred pieces. I have done a great deal of experimenting and what I've found works.

When I first started powder coating about a year ago I had different results with every piece I did. I consistently had finishes that looked like there was sand in them.

After finding my way to the forum here, everything I read about this problem was blamed on out gassing, contaminated powder, or the oven was sucking in dust. I had a hard time believing my problem was out gassing because I use extruded/ machined aluminum. To me out gassing sounded like it would be a problem with cast aluminum not extruded. But, I went ahead and started preheating the parts as suggested, but it didn't seem to help much, as I was still getting mixed results. I began to think the problem was contaminated powder or that my oven was sucking dust. I was close to giving up on powder coating after all attempts to rectify the problem had failed. I had pretty much decided to start sending my stuff out to an anodizing shop when it dawned on me what the problem was.

The solution actually came from my experience with Mold Making. A little over a year prior to getting started in powder coating, I was making some injection molds that I inject with rubber. After making the molds the last process is to media blast the mold. In the beginning I found that I would have to inject the mold 6-7 times to clean the sand (glass bead) from the pores before I got a clean part. Then by accident one day I spilled some oil on a mold I had just sand blasted and decided to boil it in hot water to clean it up. I found that I got a virtually "sand free" mold from the very first injection. At that time I didn't know that the pores in aluminum expanded when it was heated up, but I did make the connection with the mold being almost perfectly clean after boiling it.

I'm not sure why it took so long for me to put 2 and 2 together with the powder coating but when I did it made perfect sense. What was happening was the media was getting embedded in the pores with the media blasting, then when the aluminum heated up and the pore expanded, the sand was push out. ( Probably during cooling.)

Many people will likely say the problem was with the part being dirty to begin with, but I assure you I was overly couscous with shooting a perfectly clean part. I did experiment with several process of cleaning the part after media blasting (also tried diffrent presssures with the blasting) but nothing gave me a perfect finish. One of the processes went like this: Boil the part, then scrub with a clean brush while it was still hot, pre heat in oven, then brush down again and spray off with acetone (carburetor cleaner). With all that cleaning and pre heating I was still getting mixed results. Finally I just stopped blasting my parts and the problems have completely gone away. Currently, I do NOT pre heat any aluminum parts before powder coating, I have a 100% perfect out come on EVERY piece I do and I do allot!

[sdold]

Thanks for taking the time to write that up. Did you try any other media, like aluminum oxide, to see if you had the same problem?

I sand blasted (with sand) a cast aluminum porch light that had the same effect even after pre-heating. I have wondered if it might be embedded sand, since that's what it felt like. After reading this I think that might have been what it was.

Steve

[nuttyman]

often times it is necessary to blast Al to strip other finishes or to get rid of small scratches, etc. It is absolutely necessary to clean off the residue left by blasting media prior to coating as there is residue left after blasting. If the part is absolutely clean after blasting you will not have this problem. Whetever works for you I guess!


nutty

Raw metal don't need blasted - clean very good and shoot !!!!

[mtumbleson]

Originally Posted by sdold

Did you try any other media, like aluminum oxide, to see if you had the same problem?
Steve

No all I ever used was glass bead. I will concider it though...

[mtumbleson]

Originally Posted by nuttyman

often times it is necessary to blast Al to strip other finishes or to get rid of small scratches, etc. It is absolutely necessary to clean off the residue left by blasting media prior to coating as there is residue left after blasting. If the part is absolutely clean after blasting you will not have this problem. Whetever works for you I guess!
nutty

Excuse me if I take your post to be arrogant and condescending. As I stated in my first post I went through all the proper cleaning techniques and then some to assure a clean free surface before shooting.

If your read my post with some care you see truth to what I'm saying. I know beyond a shadow of a doubt that what I posted is true, I spent countless hours figuring out the problem. I will not argue with you about weather or not my conclusions are false or misinterpreted.

[mtumbleson]

Originally Posted by customandsound

Raw metal don't need blasted clean very good and shoot !!!!

Yes sir I agree, but there is always some finishing needed when parts come off the mill.

Media blasting is always the quickest and easiest to de-burr and prep the surface for powder coating. Problem is it's hard to sell a product with glass media in the finish.

[Popeye]

mtumbleson,
Sounds like you've got a system that works..
For smaller parts we usually use a vibratory tumbler, takes longer but it's less expensive than blasting.

[nuttyman]

sheeesh! perception is a subjective thing I guess! my wife often tells me that I tend to be crass & insensitive when dealing with others. is this something left over from childhood?! I don't like arguing either.


nutty

[mtumbleson]

Originally Posted by Popeye

mtumbleson,
Sounds like you've got a system that works..
For smaller parts we usually use a vibratory tumbler, takes longer but it's less expensive than blasting.

The tumblers are a great idea, but I would need too many of them for it to be worth the investment. Right now, I simply de-burr with a 90* die grinder and sanding pad, hand wash in warm soap and water, spray down with acetone and shoot.

I have the system down, it requires very little prep and the pieces are turning out great. I find that the powder coat forgives "sins" that anno won't.