# Finish Chemistry Reference

Combined reference for finish compatibility, cure protocols, and defect diagnosis.

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# Finish Compatibility Matrix

Rules for layering finishes. "Over" means applied on top of a cured layer.

## The Golden Rule
**"Fat over Lean"**: More flexible finishes can go over harder finishes, but rarely vice-versa.
**"Polar over Non-Polar"**: Generally risky.

## Matrix

| Base Layer | Top Coat: Nitro | Top Coat: Poly | Top Coat: Shellac | Top Coat: Oil | Top Coat: Waterborne |
| :--- | :--- | :--- | :--- | :--- | :--- |
| **Raw Wood** | YES | YES | YES | YES | YES |
| **Shellac** | YES (The universal sealer) | YES (Dewaxed only) | YES | NO (Oil won't penetrate) | YES (Dewaxed only) |
| **Nitrocellulose** | YES (Melts in - burn in) | NO (Adhesion failure) | YES | NO | NO |
| **Polyurethane** | NO (Crazing risk) | YES (Scuff sand) | YES | NO | YES (Scuff sand) |
| **Oil (Cured)** | NO (Unless fully cured 30+ days + Shellac barrier) | NO | YES | YES | NO |
| **CA Glue** | YES | YES | YES | NO | YES |
| **Vinyl Sealer** | YES | YES | YES | NO | YES |

## Special Notes
- **Oily Woods (Cocobolo, Rosewood):** Must be sealed with **Shellac** or **Vinyl Sealer** before applying Nitro or Poly, or the finish will not cure.
- **Waterborne:** Extremely sensitive to stearates in sandpaper and wax in shellac. Always use dewaxed shellac.

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# Cure Protocols

Time-to-buff and Time-to-recoat schedules.

## Environmental Factors
Standard conditions: 70 F (21 C) at 45% RH.
- **Cold (<60 F):** Double all times.
- **Humid (>65% RH):** Risk of "Blush" (milky clouding) in Nitro. Do not spray.

## Schedules

### Nitrocellulose Lacquer
- **Flash Time:** 15-20 minutes.
- **Recoat:** 1 hour.
- **Cure to Buff:** 14-21 days minimum. (Smell test: if it smells like solvent, it's not done).

### Polyurethane (Wipe-On)
- **Flash Time:** N/A.
- **Recoat:** 4-6 hours.
- **Cure to Buff:** 3-5 days.

### Tru-Oil / Danish Oil
- **Soak in:** 5-10 minutes.
- **Wipe off:** Immediate.
- **Recoat:** 12-24 hours.
- **Cure:** 7 days.

### Shellac (French Polish)
- **Flash:** Instant.
- **Session Gap:** 24 hours between heavy spiraling sessions.
- **Cure to Hard:** 3-5 days.

### Waterborne (Target / StewMac)
- **Flash:** 30 minutes.
- **Recoat:** 1-2 hours.
- **Cure to Buff:** 5-7 days (Cures faster than Nitro).

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# Finish Defect Diagnosis

## Symptoms & Causes

### 1. Blush (Milky/Cloudy Haze)
- **Cause:** Humidity trapped in the film. Cooling solvent condenses moisture from air.
- **Fix:** Mist with "Blush Eraser" (Retarder solvent) to re-flow and release moisture.

### 2. Fish Eye (Craters)
- **Cause:** Silicone contamination. (Did you use Pledge or WD-40 nearby?).
- **Fix:** Seal with Shellac. Add "Fish Eye Eliminator" (silicone additive) to lacquer (Last resort).

### 3. Orange Peel (Bumpy texture)
- **Cause:** Fluid pressure too low, viscosity too high, or gun too far from surface.
- **Fix:** Level sand and spray wetter/thinner coats.

### 4. Runs / Sags
- **Cause:** Too much material, vertical surface, gravity.
- **Fix:** Let cure hard (24h), scrape flat with razor blade, sand, re-shoot.

### 5. Witness Lines (Map contours)
- **Cause:** Sanding through layers of color/clear.
- **Fix:** Spot repair color, clear over everything again.

### 6. Crazing (Cracked glass look)
- **Cause:** Thermal shock (Cold guitar -> Warm room) or incompatible layers.
- **Fix:** Amalgamator (for Nitro) or Strip and Refinish (Poly).
