Sanding stains with circular

[kromalum]

Hi folks,

I'm still having trouble with that grill I'm working on. I don't get enough time to work on it so I'm trying to solve the problems slowly.

After buffing this weekend I noticed that there were some really really fine marks in the metal. I could see them with a magnifying glass or with a digital picture. It looks to me like possibly scratches from a DA but too fine and no real pattern. One of the guys who polishes aircraft trim said to only use a circular on aluminum.

So I went back to my circular. Here's the problem. I started getting what looked like color stains. When this happened to me initially a few weeks ago I thought at first the piece was corroding from rough sanding and leaving it too long. Then I read a product sheet on isopropyl alcohol which I was using to clean between steps and it said that it was incompatible with aluminum. So I thought I had checmical discoloration. Finally, I thought maybe the hook backing on my sanding backup plate may have dipped too close and burned the plastic into the metal because it was hard to get out.

I sanded like crazy and it appeared the only thing that got the marks (or discoloration) out was some kind of really fine emery cloth from my "assorted" sandpaper box. The discoloration wouldn't buff out with a wheel and black emery, and buffing just looked dull in that area. After the emery sandpaper and progressive grits and the discoloration was removed it then buffed up nicely (but still with the white streaks we were talking about before).

This weekend when I decided to put some 800 grit on the circular and go back over the already polished area to see if I could get out the fine pattern I wound up getting that staining color again. My circular sander is a 6" disc running 2000 RPM which is probably fast. But I knew that going in and I just glided over the surface with no pressure at all moving at a medium speed....and I only went over it once - back and forth! When done I had this big dark colored spot and something that looked like a heat burn around it. In fact, as I moved to the next area (a spot between the lettering on the grill) I could watch the discoloration appear before my eyes.

Has anyone ever experienced this before?

I knew I was going to be in for a horrible time trying to remove the spot but just for a gas I put some mineral spirits on the part and ran over it again with the sander and it started to pull some of the dark spot out which I thought odd because the mineral spirits or alcohol or soap and water on a rag won't take it out. Of course my sandpaper clogged in two seconds because it was dry paper.

I was also wondering if heat previously applied to the back of the part could cause this? I have heard that heating aluminum can cause problems for anodizing too. I know that it was heated at one point to remove some old paint in the lettering. Or am I just running my rpms too fast? Would I be better to use an aluminum cutting fluid and slower speed?

[jimcarry]

Re: Sanding stains with circular

There is some really good advise and tips on polishing aluminum in this forum.
I have some accounts that I polish aluminum for you want to make sure you clean
the part good between steps I use Dawn dish washing liquid and hot water but the
parts I work on are small enough to do that . The brown spots sound like burns
you maybe applying to much pressure or just simply leaving the wheel in one place
to long .after you final polish and cleaning you should be able to buff out any swirl
marks left .

[kromalum]

Re: Sanding stains with circular

I agree, I've found a lot of good information in the forum. One thing that gets me though is how so many people have such different ways to polish. For example, many people here say that the Caswell formula of starting with a sisal and aluminum on emery is too rough. Caswell follows with brown, which many people have had trouble with on aluminum, me included. Finally, Caswell says to use progressively finer compounds with finer wheels, but there was one chap with a 'sticky' in the forum who polished his motorcycle with good results doing the opposite - and finishing with a sewn wheel I believe. I wish there were a definite step that worked, but I guess there are too many variables in this kind of work.

As for burns, I'm thinking that may be the trouble. I'm definitly not using too much pressure or staying in one spot too long with this piece. Particularly with 800 grit. I'm just gliding over the surface trying to hold the sander up. I'm also using a homemade 1/2" foam interface pad for fine sanding between the backup pad and sandpaper to eliminate chatter marks - which were a problem before.

I've heard other people on the forum using alcohol to clean between steps, but I'm guessing that is denatured alcohol? I use Palmolive and hot water but find that I really have to scrub hard to get the very tiny compound wax spots off - and that scratches this soft piece. Alcohol gets them in one wipe.

Anyway, I'm hoping it is a burn because I can fix that by sanding slower RPM with cutting fluid. Although, does the very strange circumstances under which these marks appear say anything about the grade of aluminum?

[jimcarry]

Re: Sanding stains with circular

Well it may not be just the kinds of work but the different grades of aluminum like you say .
You might try going to a palm sander instead of a orbital . But my thoughts are that every one dose things different and you just have to find the way that works best for you. Like me I use a light weight palm sander for all my sanding work till it come to the final steps then I wet sand by hand in warm soapy water. When I use the dawn soap and hot water to clean with I soak the parts in for some time till I can get my hands in with out burning them then I go to final buffing no compound

[pickleboy]

Re: Sanding stains with circular

i only machine sand to 400 grit myself and then finish out with hand wet sanding.its slow but effective. it also may help to do the wetsanding with some sort of oil like wd-40 or water with dish soap in it. once i get to 600 grit i wheel the part from there using emory and vented wheels. then on to the white on spiral sewn and then loose with white. the emory stage should take out 98% of your scratches and the white will remove the swirls. to clean between steps i use aerosol brake cleaner as it melts the compound off quick. hope some of this will help.