Polished enclosure resistant to etching?

[Johnny Lemonhead]

I bought a pre-polished enclosure for etching, so I obviously didn't want to sand the surface and ruin the finish. After ironing the PnP and masking the sides, I started the etching process with ferric chloride as I did before with a regular home-sanded enclosure.
First I thought the ferric chloride-to-water-ratio was wrong because nothing happened for a long time, but after adding some more FC to the mix, I began to think the brushed aluminum might be etch resistant.
I looked closely at the parts where I retouched the design by scraping the surface with an x-acto knife during the masking process, and those did etch.

Does anyone have any experience etching these enclosures, or is it a known fact that they cannot be etched?

Thanks.

[chi_boy]

Are you sure it isn't clear coated?

[Johnny Lemonhead]

Well, it only said "polished" when I ordered it and it didn't appear to have been coated. Felt like finely brushed, buffed aluminum.

[nosamiam]

I would guess it has been coated. There is a chemical that can be sprayed on bare aluminum (or it can be dipped in it) to prevent corrosion. It's what they use on polished rims for cars. It's visually undetectable. Maybe try paint thinner? Ultra-fine steel wool?

[R.G.]

I would guess it has been coated. There is a chemical that can be sprayed on bare aluminum (or it can be dipped in it) to prevent corrosion. It's what they use on polished rims for cars. It's visually undetectable. Maybe try paint thinner? Ultra-fine steel wool?

I think of this esoteric chemical as "lacquer". I'd try wiping it down with lacquer thinner, perhaps also acetone.

It is possible that it's coated with catalyzed urethane, but unlikely.

[Johnny Lemonhead]

I would guess it has been coated. There is a chemical that can be sprayed on bare aluminum (or it can be dipped in it) to prevent corrosion. It's what they use on polished rims for cars. It's visually undetectable. Maybe try paint thinner? Ultra-fine steel wool?

Yeah, I will have to remove all the masking, sand it down and do the whole thing from scratch. Very annoying, especially because the higher price of the polished enclosure was supposed to spare me from applying time and elbow grease and not the opposite...

Anyone else planning on etching a pre-polished enclosure should take this to account.

[Johnny Lemonhead]

I think of this esoteric chemical as "lacquer". I'd try wiping it down with lacquer thinner, perhaps also acetone.

It is possible that it's coated with catalyzed urethane, but unlikely.

I now recall wiping the surface with acetone prior to ironing the PnP, so it's resistant to that as well. Does that make sense?

Thanks for the help.

[R.G.]

Acetone doesn't remove lacquers all that well. There are several "lacquers". The specialize lacquer thinners are better. But sand/steel wool/polish is surer - if slower. Silicon carbide particles are a known solvent for all coatings softer than sapphire and diamond.

Don't laugh - vacuum deposited diamond coatings do exist. 

[Johnny Lemonhead]

Thanks for the information, R.G.

I have no choice but to sand it all down. The small grooves I made with the x-acto knife, during the touching-up of the masking, let way to the etchant that dug deep into the surface. I might also sand the sides as well for uniformity.