If you've got at CNC - or sign writing kits for a regular router, or even free hand sign routing - often you'll have the desire to paint the milled out area.

The problem is, how do you get it done without getting paint everywhere? You could just spray/brush paint on everywhere and sand back, but that has the issue of burning through sand paper pretty quickly, and depending on the detail of your carving can result in wood chipping off.

The solution is spray masks!

So what are spray masks? Spray masks are (generally speaking) a vinyl based product with adhesive on one side. They cut easily with scissors or a knife, mill pretty well (and don't melt easily), and resist most (but not all!) solvents found in spray paints and the like, which helps prevent any bleed through.

They're not just for spray paints - you can get great results brushing on acrylics too.

I've only used two products from Aslan, S79 and 85G. With the appropriate level of prep work, they're identical in function.

Other brands include Avery and Oramask

Step 1: Preparation

Paint masks typically contain a reasonable - but not double-sided-tape level - adhesive. As such, they stick really well to smooth surfaces, but less well to porous surfaces.

Sand your sign blank to at least 180grit, 220/240grit is a bit better.
Make sure you dust/vacuum up any dust afterwards too, thats just going to inhibit adhesion
Apply some sort of sealer over top
This doesn’t have to be your final top coat, but it can be. If you’re unsure of what to go for here, try dewaxed blonde shellac. It won’t impart much of a colour to the wood, and being dewaxed you can stick just about any finish over the top.

I’d advise against BLO or tung oil here as they don’t really seal the surface that well.

If you’re using shellac, a 1.5-2lb cut is enough, do two coats, sand with 220/240g after the second coat.

Step 2: Applying the Mask

Cut the vinyl sheet to the size of your blank, then peel back a little bit of the backer, and using a scraper or roller, slowly pushing it onto the surface. If you’ve ever applied contact to a book, its exactly the same process. You want to have a smooth, bubble free layer. Apply a decent amount of pressure, rolling/scraping over after you are done - you want that sucka to stick.

I've used/tried two different rollers - a speedball brayer and Fastcaps SpeedRoller. Honestly I find the brayer a little better for actually getting the vinyl down, then switching to the SpeedRoller to apply as much pressure as possible. You can get away with either.

Should I wait to mill?
If you look at the TDS of many paint mask products, they’ll list the immediate tack as well as the tack after 24hr or even 7 days. If you’re having issues with a proper paint mask, waiting a day to carve after applying may help you. Make sure you’re also applying the mask within the appropriate temperature range!

Step 3: Milling

In general, milling can go now ahead with little changes from normal.

However, there are a few important things to pay attention to

Dust collection - the bristles on most dust boots will likely pull the mask off - raise it up above the surface so its not touching. You won’t get as good dust collection, but it should still trap the dust. I had no issues when I raised it ~5mm above the surface, and had the vacuum on
Bit selection. “Straight bits”, such as V-bits are "fine" - not great, but fine. Downcut spiral bits are better (and if you’re doing a roughing/clearing pass, I highly recommend downcut bits so you don’t have to sand afterwards anyway), but avoid upcut bits. They will likely pull the mask away.
Really critical shallow details? Make sure to factor in the thickness of whatever mask you’re using when you’re zeroing. The Avery and Aslan stuff is very thin, but some people use contact paper which can be thicker

If you're using a regular (non-CNC) router, you may find you need to elevate the router on spacers or something to avoid catching the edges of the mask.

Super tiny/crisp detail works out just fine if you’ve adequately applied your mask. The crown in the Gondor emblem measures about 12mm across.

Step 4: Paint Prep

More prep work?! With everything milled, apply one or two sealer coats of shellac to the milled area.

But why would you add more work? If you're using wood, the capillary nature of wood means the paint may bleed through the grain. You can have a perfect spray mask, but if the paint wicks through the grain, you're out of luck.

Or if you're using a water based product like acrylic paint, you might end up with some very raised grain as you paint.

Again, this can be a top coat product like polyurethane, but this is where dewaxed really shines - two coats will be dry in about 40minutes, and you're right to paint. Dewaxed shellac is universally compatible too, so you don't have to worry about any interaction with.. anything!

Step 5: Painting

Spray paints or acrylics applied with a foam brush to dab on work just fine. You could probably get away with a soft paint brush too,but be careful not to lift the edges with the brush. I think airbrushing could look pretty fantastic too.

Paint masks are designed to be solvent resisitive, so you won't have issues with the majority of spray paints - there is always one outlier that is going to end up being weird.