Intricate Masking

We have a client who needs some prototypes painted, and they include some small lettering (~1/4" - 5/16" high) which I think will need to be masked off. The final product is a plastic handle with overmolded rubber grips, and the plastic will be showing through the rubber to create the text. There are a mix of parts we need to paint, but some will have actual rubber grips and some will just be all SLA with the rubber only represented by color (needing masking). My preference would have been to just use a decal, but unfortunately, we were contacted after the parts were already ordered so the letters are recessed; there isn’t a flat-ish surface (the handle has some contours, but is mostly flat in that area) for me to rub the decal on.

So it seems we have two choices: Get out the tiny brushes to hand paint inside the recess and prepare for cramped hands or figure out a way to mask the recess efficiently and accurately. I’m hoping for the latter, and my idea was to get cut the letters out of tape (the font fortunately is a stencilish font with the centers of Os etc. connected) and apply this to the handle for masking. I imagine it being cut like they cut vinyl for signs, on a plotter like machine. I’d actually go to a sign shop, except I don’t imagine vinyl would work that well for masking, mostly because it’d stick too well to the paint and rip it off. But I don’t know where to go for that, or if it even exists.

Does anyone know of a place I could get basically a stencil made out of tape? Or does anyone have any other ideas for how to mask this efficiently?

You have already thought of my suggestion. I would talk to a sign shop and see what they think about the vinyl idea. I would imagine it would work out well for your needs.

Is the finished surface a rubber material? And on a contour? vinyl paint mask doesn’t stick well to rubber, and doesn’t conform well to complex curves. The size of your copy is also going to make it a challenge, but you could give it a shot. Any sign shop in your area has, or has quick access to what you’re looking for. Also, make sure you peel that mask while the paint is wet. Otherwise, you have to cut into it which is a terrible scenario on 1/4" copy.

Here’s a trick that we have used in the past. Spray the recessed letters, when dry, sand and prep the high surface around the letters. Spray the recessed letters with either a petroleum jelly mold release or a thinned mix of petroleum jelly and mineral spirits. Carefully clean the surface around the letters and paint – the petroleum jelly will act as mask. Clean out the mask with mineral spirits.

Another trick is to paint the everything including the recessed letters. Wipe the high surface with petroleum jelly and then paint the recessed letters with a water based airbrush paint. The over-spray will clean off with mineral spirits.

Thanks for the quick responses. I’m getting a sample of some vinyl scraps to see how that works out, perhaps I was too pessimistic about that.

The petroleum jelly idea sounds very interesting. Do you ever have a problem with paint not sticking due to any residue left from the petroleum jelly (meaning it wasn’t cleaned sufficiently)?Does wiping with a dry or mineral-spirits-damp cloth do the trick? I’d also be a little worried about rubbing mineral spirits on my final paint, but I guess you just have to be really sure it’s dry?

No problems with paint adhering as long as you clean well. Mineral Spirits is fine with acrylic lacquer or polyurethane paint – DO NOT USE IT ON KRYLON or any enamel spray paint in cans.

If you’re worried do a test on something first.

There are all sorts of shops out there (Gleicher Manufacturing comes to mind), that convert VHB.

I’m sure you could contact one of those shops and have them whip out a stencil of something a little less permanent.

For future reference, instead of having the rubber represented by color, you can do an “overmolded” SLA where both a rigid and a low duro (~20A - 80A) SLA resin are built simultaneously… emulating a rigid plastic and elastomeric overmold.

Alot of the higher end SLA shops are doing them now, and for appearance and “feel” models, they can be great.

Instead of petroleum jelly, you could try a liquid mask like faskolor mask

Paint the whole thing the color of the overmold
spread a thin layer of liquid mask onto a flat surface and roll the model in it / carefully “Squeegee” it on the outer surface
let it dry
paint the letters
peel the mask off

This stuff is usually used for airbrushing so lacquer may eat through it. I did use a similar product ages ago with lacquer

Thanks for your help. We got the parts in that we’re supposed to paint (the customer was supplying them - though we do design work for the customer we also do odd jobs like this, painting parts we had no part in creating), and it turned out the recess was only .001" deep in CAD and therefore didn’t come out in the SLAs. So the issue I had disappeared.

However, I did test the vinyl and petroleum jelly beforehand. The vinyl worked really well. I got a sample of “removable” low tack vinyl which came off just fine, partially because it was only on for a couple hours. And the edge quality was good as long as I made sure the edges were firmly pressed (but that’s like any masking). So for intricate masking vinyl is a good solution.

I tried the petroleum jelly method as well, though not exactly your instructions. I used a thermoformed tray with a recycle symbol as my sample recess. It worked pretty well, though the edges weren’t quite crisp. I didn’t dilute the petroleum jelly, so that may have contributed to it, and the edges were very rounded so maybe I couldn’t get a crisp wipe. Also, the paint we had on hand was oil based enamel. It’s not in a spray can, but I was still betting I’d have a problem with it. However, there didn’t appear to be a problem at all. I just made sure the paint dried enough, then the only paint that was affected had petroleum jelly underneath. So overall a cool method if needed in a pinch - maybe with some practice I could get the edges cleaner.

Thanks for your help everyone.