A spray gun fails slowly and then all at once. The slow part is cheap to prevent: a few parts, a few minutes between sets. The sudden part is a 20-minute teardown with the crew standing in a half-sprayed room.
The 90 seconds between sets
- Air-purge through the mixing chamber until nothing more comes out.
- Look at the chamber face. A pebble of cured foam on it will bend the pattern; pick it off.
- Check the front cap o-ring. Hard, shiny or flattened means replace.
- Grease the side seal bores.
- Check the air cap for buildup.
Skipping it saves a minute and costs the teardown later.
What to replace and when
| Part | Replace |
|---|---|
| Mixing chamber | Every set; every two sets if it is clean and the pattern is sharp |
| Side seals (cartridges) | Every chamber change |
| Side seal o-rings | Every chamber change |
| Front cap o-ring | Every chamber change |
| Fluid screens, A and B | Every chamber change, sooner if pressures drift apart |
| Check valves, A and B | About every 10 sets, or any time one pressure wanders |
| Air cap | When the air pattern goes uneven |
| Piston o-rings and handle seals | When the gun gets slow to open or close, or leaks air |
Keep a service kit on the rig with two of everything in this table. One kit is a day of work; a missing o-ring is a day lost.
Symptom to part
| Symptom | Check first | Then |
|---|---|---|
| Spitting, pulsing, pattern breaks up | Side seals and chamber | Pressures A versus B, screens |
| Crossover (foam curing inside the gun, one side backing up) | Side seals, chamber ports | Check valves, pressures |
| Pattern off-center or one-sided | Chamber face buildup, air cap | Chamber scored on one port |
| Pressure gap between A and B grows while spraying | Fluid screen on the low side | Check valve, drum running short |
| Leaking at the front | Front cap o-ring | Chamber seating |
| Gun slow to trigger or leaks air | Piston o-rings, air seals | Air supply and lubrication |
Chamber size
The mixing chamber sets the output. Fusion AP round-pattern chambers run from AR2020 (small, for detail work, rim joists and low-output rigs) through AR2929, AR3737, AR4242 and AR5252 up to AR6060 for big open walls and roof decks on a rig that can keep up. Too big a chamber for the rig gives low pressure at the gun and a rough pattern; too small slows the crew and runs hot. Match the chamber to the proportioner's output and the hose length, and keep two sizes on the truck.
Graco OEM or Sprayco
USAFoam stocks both. Graco OEM parts are the factory parts. Sprayco parts are made to fit Fusion AP and PC guns directly: chambers, side seals, o-rings, screens, check valves and rebuild kits, at a lower price than the branded part. The parts lookup shows the gun diagram, the Graco number for every position, and the Sprayco part that fits beside it, with live stock at your branch.
A rebuilt gun sprays like a new one. A new gun with old side seals does not.