Enclosure marks/staines after polishing

[slashandburn]

I've had this issue in occasion before and for the most part just ignored it, you'd have to be really looking for it to notice. Im not aware of any pattern to indicate the cause. This box I even avoided wet sanding completely. I wasn't aware of these marks appearing until I was finished etching, painting and had polished up to about 1200 grit.

This time it's annoying me enough to bother asking how how to resolve it or at least avoid it happening again. Im suspicious it could be the sandpaper itself - the 1200 grade is a different brand from the other grades, but I have nothing to back this up besides only beginning to notice it after using the 1200.
Hopefully you can see issue in this photograph, liquid looking blotches, most visible below the labels Vol and Pre. Also a little patch to the right hand side of the big etched circle.


Anyone else had this problem? It's not a massive problem, just a bit of an annoyance. I'd quite like to establish what's causing it so I can stamp it out in future.

Edit: should be more visible in this photo

[EBK]

Those splotches are artifacts from the casting process and the alloys used.  There is a name for them, but I'd have to look it up again. 
The only way I know to avoid those splotches is to use a machined block enclosure instead of a die cast one.  Those splotches are also the reason why you wouldn't want to anodize a die cast enclosure (they would become even more prominent).

[slashandburn]

Ah okay, that put my mind at ease a little.

I'd just noticed my 1200 grit paper is marked "silicon carbide" while my other brands make no me tion of silicon. I was trying to find a way to pin the blame on that.

Cheers Eric!

[EBK]

Your other sandpaper may say "carborundum" instead, which is the same thing.  It is good stuff.

I think the term for the splotches is macrosegregation, by the way, but I'll defer to anyone with actual knowledge of the subject.

[slashandburn]

Cheers Eric.I just find it odd how I never notice it until getting down to the finer grits, and even then only on certain enclosures. Struck me as some sort of contaminant.

It's just one of those things. All that sanding and following a process that's worked almost every other time and then at the final hurdle...damn! A blemish.

I like your answer, means I can stop worrying about it and move on. Chalk it down as one of those things that cant be helped. Like when a fly gets stuck in the clear coat.

[jfrabat]

I just learned something new...  I had a similar issue with my looper.  I wanted to polish it, but it had A LOT of those marks and some darker spots.  Ended up painting the thing!

By the way, I really like your etching.  Mind sharing the process?

[EBK]

Just embrace it like wood grain -- as character, not defect.

[slashandburn]

I'm quite happy how it's turned out. Also good to know I can safely go back to wetsanding without worrying about causing these marks. Anything to keep that aluminium dust under control.

By the way, I really like your etching.  Mind sharing the process?

Hey thanks, sure thing! I'll say up front though that I don't etch very often anymore, so my process could probably use some refinement to get the results up to the level of some of the incredible work I've seen from others on here. for the most part I usually stick with waterside decals (less can go wrong and any mistakes or mishaps are easier to recover from!). This box and the other three on my desk only came to be as I misplaced my waterslide decal paper. 

I hate sanding and find it mind-numbingly tedious so step one is usually always to open a beer, preferrably of the Stout/Porter variety.
Prep the enclosure by sanding through up through the grits from 60 to about 320 - Any finer than 320 or 400 I've found the image transfer doesn't take as well - then scrub the enclosure clean with Isopropyl Alcohol or acetone (whichever is closest to hand!)

Cheap Glossy Photo printing paper has served me well for the image transfer. I've tried magazine paper and that blue press and peel stuff and had mixed results. Photo paper is cheap and has always worked for me. Print (laser), cut to size and iron on to the enclosure. Typically set my iron as hot as it goes, No Steam. Apply pressure, spent a bit of extra time on edges and corners. I probably overdo it here, 5 to ten minutes. Then dump the enclosure in water and let it soak. The photo paper peels/rubs off in layers, so soak, peel, soak, rub, soak, rub.

Hopefully you get a nice clean toner transfer with no patchy areas. If the toner didn't take on any spots you can try touching it up with sharpie or nail polish, I'll be honest I've had mixed results with this, more often than not I find if I've had to resort to this I can't get as deep an etch as whatever I use to patch up isn't quite as etch-resistant as the toner. Definitely an aspect I could improve upon.

Tape off the sides, corners any larger areas on the front not to be etched with that brown plastic-y parcel tape, 2 or 3 layers on the corners to avoid etchant seeping in. This stuff is great etch resist.

Place the enclosure face down in the ferric chloride solution (I'm not sure on concentration levels, I think the stuff I have is 40% premixed, diluted even further. Typically I'll won't use fresh stuff for enclosures - too agressive - I'll use some nasty looking old stuff that's already been used to etch pcbs. Warm it up if things are moving too slowly.). Periodically take the enclosure out and wipe down, check the depth of the etch. If im going for a really deep etch i sometimes gently scub the parts being etched with a toothbrush and dip it back in the etchant for another few minutes.

Rinse off the enclosure, peel off the tape, scrub off the toner   and remnants of the parcel tape with acetone and some steel wool and pray for the best!

If everything went to plan I'll then work up through some finer grades of sandpaper, shoot some paint, let dry, run back over with the last grade of sandpaper again. Job done. I'll sometimes at this stage hit it with a some Clearcoat.

If the Beer and Etchant gods have been kind I'll hopefully end up with something that looks like this:



I should have these and two others boxed up by tonight so I'll post the finished articles in the etched enclosure thread later! All said and done, as much as I like the results I hate the process. I'd much rather "cheat" and just use decals. It condenses that whole process down to a few simple steps:
open beer.
Sand enclosure
Clean enclosure
Apply decal
Drink more beer while Decal drys out
Spray clearcoat
Done.