How do I carve HDU sign foam: bits, settings, priming, and finishing?

What HDU is

High-Density Urethane foam board (brand names: Precision Board, SignFoam, Duna). THE professional exterior sign material: completely waterproof, zero grain, never rots/warps/cracks, carves like a dream. Common densities are 15 to 20 lb/ft³ (15 lb is standard for signs; 18 to 20 lb for fine detail and durability). It exists because wood outdoors eventually fails, and HDU doesn't.

Machining

• It cuts like butter. Almost any sharp bit works; chip load is nearly irrelevant to quality. Run FAST: it machines at several times wood feed rates, and the risk isn't tearout, it's melting/fuzz from rubbing with dull bits.
• V-carves and 3D reliefs come out flawless: no grain to fight, no tearout, fine details hold.
• Texture toolpaths (sandblasted looks, woodgrain) shine on HDU backgrounds, which is the classic dimensional-sign style.
• Dust is fine and static-clingy. Urethane dust is still a respiratory irritant, so use dust collection and a respirator, and check the manufacturer's safety data sheet.
• It's light! Workholding: tape + CA glue or vacuum; use light clamping (it dents, so pad your clamps).

Finishing (where the craft lives)

1. Prime with a high-build primer made for HDU (or quality acrylic primer). The cut surface is slightly open-celled, and primer fills and hardens it. Two coats, light sand between.
2. Topcoat: exterior acrylic latex or sign enamels. Two colors is the classic workflow: base coat the carve, then roll the raised faces a contrast color (see the two-color sign entry).
3. Edges and backs too. Full encapsulation is what makes a 20-year sign.

Pro tips

• HDU has no structure, so spanning signs need an internal frame or mounting cleats glued/bolted in (epoxy or urethane adhesive).
• Dents repair with two-part epoxy filler, sand, re-prime.
• Sell the longevity: HDU + proper paint outlasts every wooden exterior sign, so price accordingly.