Spray Gun Cleaning and Solvent Waste Decision
A shop uses a single gravity-feed HVLP gun for both solventborne primer and waterborne basecoat. At the end of each day the apprentice flushes the gun with the cheapest available solvent, blows it dry with shop air, and stores it in a cabinet. After two weeks, basecoat shows fisheyes and small gel-like particles. The shop also reports solvent waste drums filling faster than expected, and the spray booth filter log shows filters changed every 40 hours instead of the recommended 60. The gun's fluid passage is 1.3 mm and the air cap has dried residue visible around the horns.
1. Diagnose the contamination and explain how the cleaning practice contributes to it.
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The fisheyes and gel particles point to cross-contamination between solventborne and waterborne products. Flushing with a generic solvent does not properly remove waterborne basecoat residue; dried waterborne material can re-wet into gel particles when solvent is introduced, and solvent residue left in the gun can contaminate the next waterborne batch. Fisheyes are typically caused by silicone or oil contamination, which can come from shop air that is not filtered or from solvent that is not the correct type for the product.
Blowing the gun dry with unfiltered shop air can push oil and moisture into the fluid passage and air cap. Dried residue around the air cap horns distorts the spray pattern and can spit particles. Storing the gun without disassembling and cleaning the air cap and needle/seat means residue hardens in the 1.3 mm passage, reducing flow and causing uneven atomisation.
The faster drum fill and shorter filter life suggest the apprentice is using excessive solvent per flush and spraying more overspray than needed, possibly because a partly blocked gun requires more passes to achieve coverage.
2. Recommend a cleaning and waste-handling procedure that addresses the contamination and the waste volume.
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Use product-specific cleaning: flush waterborne basecoat with the manufacturer's waterborne gun cleaner or deionised water, then flush solventborne products with the correct solvent, and never share a single flush bucket between the two. Disassemble the gun daily: remove the air cap, nozzle, and needle, soak them in the correct cleaner, and clean the air cap holes with a soft tool, never a metal pick that can enlarge the holes. Blow dry with filtered, dry air and lubricate the needle packing per the manual.
Reduce waste by using a enclosed gun washer with a recirculating solvent and a settling sump, which extends solvent life and reduces drum volume. Separate waterborne and solvent waste streams and label them; never mix them, because mixed waste is more expensive to dispose of and may be prohibited. Track flush volumes per gun per day to find excess.
On the air side, add a coalescing filter and drier at the gun drop, and check the booth filter change schedule against actual pressure drop rather than a fixed 40 hours. If the filters are loading early, investigate overspray technique and gun settings before extending the interval.