reversed etch: problem with sanding paint

[poppyman]

Hello,
I've been trying reversed etch on several boxes with pretty good results...randomly.
The main concern I have, is this black gooey residue I get sometimes when I sand the box.
It doesn't happen all the time, even if I use pretty much always the same technique:

Get a deep etch
clean the box with acetone
spray several layers of spray paint et let it fully dry (about 6 hours in the sun).
then I sand block it with 1200 grit sand paper and I insist on part with just smaller bits of sandpaper, all that under running water. I change my sanding paper at least 4 times during the process.
The black goey stuff seems to appear just on the "lower" layers.


Does any one has a good advice to avoid this? (more paint drying time, different sanding technique or grit....)

Thanks in advance.

Syl

[Jdansti]

I'm not sure, but it looks like the aluminum dust from the sanding is sticking to the paint. Notice that it only appears adjacent to bare aluminum. 6 hours of drying in the sun might not be long enough for the paint to cure. It might look and feel dry after 6 hours, but you should check the instructions for the paint and see what the cure time is.

[gritz]

It might be that the paint is getting too thick in places to dry properly. It might be hard to apply it thinly, as it will naturally collect in corners and edges. It might seem hard on top, but underneath it can stay soft and that may be picking up sanding residue. I'd say try to keep each coat as thin as possible and maybe try baking each layer of paint in a warm (not hot) oven. It will stink though! As the paint surface doesn't have to be perfect (because you're sanding it afterwards) maybe try heating up the piece with a hot air gun and spray thin coats. They should set pretty much immediately without building up in the corners.

Try on a scrap piece first!

Or perhaps a two part epoxy colour (that sets by chemical reaction rather than evaporation) would work. Messy though, I guess.

Nice effect by the way.  

[deadastronaut]

as jon says looks like aluminium is sticking to the paper, use soap (washing up liquid) when sanding...warm water. 

[poppyman]

Thanks everyone!

So, the box is currently is an acetone bath. I 'm going to scrub off the paint and give it another go.

Thinner layers, more drying time (even if the can says 1 hour!), and warmsoapedn water sanding. I can't really bake the paint since I'm on a year trip abroad and DIY-ing is already very complicated!

I'll post the result in the etched enclosures photo section

cheers!