Spray Gun Crossover: Why A and B Cross in the Gun and How to Fix It

You pull the air cap and find gel in the air passages. Or the muffler is crusted, or the gun that sprayed fine yesterday is locked solid this morning. That is crossover: A (ISO) or B (resin) material getting somewhere it should not be, usually into the gun's air section, where it meets the other side and cures.

The good news is that the causes are well documented. Graco and PMC list nearly the same short list in their gun manuals, and the repair lives in one place: the fluid section.

What crossover looks like

Graco's Fusion AP and Probler P2 manuals call it "A and/or B fluid in gun air section." PMC's AP-2 manual calls it Iso and/or Resin in the gun air passages. Common signs:

• Material in the air cap passages, the muffler, the exhaust or the handle.
• Mist or drip from the nozzle when the gun is closed.
• A gun that hesitates, then snaps open, or will not open at all after sitting.
• Pressure imbalance at the machine, or a pattern that looks off ratio.

Why A and B cross in the gun

In an impingement gun, the side seals seal each material port against the mix chamber. When that seal fails, pressurized material slips past and into places it does not belong. Both Graco and PMC list the same causes:

1. Damaged side seals. Cut, worn or scratched seal faces.
2. A scratched or grooved mix chamber. A scored chamber cannot seal against even a new side seal.
3. Damaged side seal o-rings. A cut or swollen o-ring behind a good seal still lets material bypass.

Two field habits make it worse. First, tightening the air cap or front with the material valves open forces material past the side seals. Graco's Probler P2 manual is blunt about the related mistake: removing the side blocks without shutting the material valves leaves the gun encased in urethane. Second, swapping parts or solvent between sides. Graco warns never to interchange A and B wetted parts and never to use solvent on one side that was contaminated from the other.

Gun by gun reference

For exploded diagrams of each of these guns, open the Gun Parts Finder.

How to fix crossover, step by step

1. Follow the pressure relief procedure in your gun manual, close both material valves and flush if needed.
2. Remove the front end, the mix chamber and both side seals.
3. Inspect the side seal faces and the mix chamber sides under good light.
4. Replace both side seals and their o-rings as a set. Replace the mix chamber if it is scratched, since it will ruin new seals.
5. Clean every trace of material out of the air passages before reassembly. Graco says to replace damaged parts before reattaching the fluid manifold.
6. Service the check valves and screens while you are in there (next section).
7. Lubricate o-rings with the grease your gun maker specifies and reassemble. Keep A and B parts separate the whole time.

Screens and check valves

Each side of the gun has a check valve with a filter screen (PMC uses a screen screw). When one packs with debris or cured material, that side starves. Graco lists plugged check valves and impingement ports as causes of pressure imbalance, and PMC lists "dirty screens" as a cause of spray pressure imbalance. An unbalanced gun sprays off ratio and puts extra stress on the side that is still flowing.

• Clean screens in compatible solvent or replace them. Screens are cheap; downtime is not.
• Inspect the check valve o-rings and ball while the valve is out.
• Label your A and R parts. Mixing them is how cross-contamination starts.

Purge air and daily prevention

On air purge guns, air clears the mix chamber when you release the trigger. PMC's Xtreme manual notes that purge air passes through the spool valve and a check valve on its way to the gun block. Graco lists a damaged front o-ring as a cause of reduced purge air on the Probler P2, and the Fusion AP has a cleanoff air valve for front-of-gun buildup. Weak purge leaves material in the chamber, and wet purge air feeds ISO buildup at the gun.

Daily habits that stop crossover before it starts:

• Grease the gun at shutdown. Graco says to grease the Probler P2 nightly; on the Fusion AP, grease until mist shows at the nozzle, two shots maximum.
• Keep air at the gun inside its rated range while triggering, not just at rest.
• Keep the rig's purge air dry and drain the compressor daily.
• Watch the gauges. A growing imbalance is often the first sign of a plugged screen.

Still chasing it? Our equipment troubleshooter walks through crossover for the Fusion, Probler P2, PMC and Carlisle guns and ends with exact part numbers. Side seals, screens and rebuild kits are in Parts and Rebuild Kits.