Spots on Aluminum

[scocklin]

http://www.caswellplating.com/bbs/album_pic.php?pic_id=534

My son agreed to re-anodize a friend’s paintball marker and ran into some trouble. I have since had to try and bail him out on this one. It is a problem I have not encountered before and I’m wondering it could be the aluminum alloy causing the problem. I re stripped the part polished, cleaned in Caswell degreaser, and de-smutted in Caswell de-smutting agent performing a rinse with distilled water after each step. I noticed some spots that where shinny and seemed to lie over the top of the aluminum. I tested them for continuity just to check if I had gotten the previous anodization completely off and apparently I did.

I anodized and dyed the part and the spots absorbed the dye. I have enclosed a picture (click on the above url) that demonstrates the problem. Any help or suggestions would be greatly appreciated.

[sswee]

It appears to be a poor mix in the cast. I've had similar problems with some automotive parts. I plan to strip, repolish and anodize without using the DeOx/DeSmut to see if thats what is causing the problem to show so bad. Was the marker polished originally or matte finish?

[acidrain]

That looks like a Timmy frame. Most paintball guns (except Tippmanns) are milled from billet aluminum... usually 6000 series. I doubt it is the aluminum.
It looks like spots from the prep process. I noticed you didn't de-smut sfter the etch process. I always do that regardless of alloy or lack of apparent smut.
Instaed of caswells etch, try red devil or drano brand chrystal pipe cleaner (sodium hydroxide). 1 can mixed with 2-1/2 gals. of water @ 65F.
Follow with caswells de-smut @ 110F.
Polish or beadblast, then degrease in lacquer thinner, then caswells SP degreaser @ 140F.
Ano immediately after the finish and cleaning steps.
Distilled or RO rinse between each step (of course).
After rinsing the ano electrolyte, submerge parts into a bucket of baking soda/water neutralizer.
Immediately rinse and dye after ano.
Hope that helps.

[scocklin]

Thanks for the replies, I did everything you listed acidrain. I have used Red Devil sodium hydroxide in the past and I find it throws off more fumes (NaOH fumes are asthma inducing) while it’s working on the aluminum. Is it a stronger concentration vs. Caswell (sodium hydroxide) stripper?

I tend to go with sswee’s theory seeing as not all of the parts from the marker had spots on them after the preparation work and the clean parts look like different alloys. As I said before; the spots on the raw post prep metal prior to anodization looked like they where shinny layers of metal lying on top of the aluminum piece the spots ohm out telling me all of the anodization was removed. I’m hoping either Fibergeek or M_D will have a look at this problem and enlighten me.

Yes the part was originally polished my next step is to strip and polish all of the parts in question prep them and re-anodize them. Thanks again for all of the replies!

[Fibergeek]

Although either Acidrain or Sswee could be right, it could be as simple as uneven surface texture, smooth texture will anodize differently than matte texture.

Continuity testing is pretty unrelable; you can never be sure that you haven't dug below the offending surface.

[acidrain]

Oops, just read you post again and noticed you DID desmut. Sorry for the long post.
I can tell you for sure... it's not the aluminum.
Paintball guns is all I work on, and on occasion I've run into blemish problems. Each time it's been a matter of alternating stripper and de-smut until the blemishes are gone.
GL, -Acid

[sswee]

After running into what looks like the same thing, I remembered this post. At first the spots appeared to be poor mix in a cast. Someone said the guns were made of billet. The spots are hot spots from machining. The material work hardens in spots changing the density of the metal. I have seen this many times in the machine shop but didn't put the two together.

[acidrain]

Thank you for that info sswee.
I've run into a few pieces that had obvious blemishes under the existing ano, that were very difficult to make disappear.
I'm stumped though... how were the original anodizers' able to hide these blemishes even with lighter colored dyes?
Were you able to diminish the effects of those blemishes?

[sswee]

I'd like to know myself why and how some defects are brought out or covered up. The billet parts I've had blemishes on, I stripped, resanded, prepped and etched to fix the problem. The parts I've had to rework are either glass beaded finish or highly polished. I can't remember one that had a finish that was in between the two. I haven't had to deal with a close dimensioned part with this problem. Probably because they are machined differently to aquire the dimensioning.

Just my thinking. The original anodizing covering of blemishes may be due to voltage anodizing and the higher CD that they run at.

[scocklin]

Sorry for the delay posting, I've been out of town working my real job. I finally remedied the spot problem on the marker by polishing it using black, brown and green compounds. The marker anodized fine with out spots after the polish job. Still not sure what the problem was but I have informed my son to please stay away from Timmy’s for now until we get a little more experience with various alloys.