Thinner is treated as a commodity on many sites: whatever is in the store gets poured into whatever is being sprayed. It is one of the cheapest ways to destroy an expensive coating system.
Thinner does more than reduce viscosity
A manufacturer thinner is a blend chosen for the specific resin chemistry, balancing solvency, evaporation rate and flow. The evaporation profile is the critical part. Solvents that flash too fast leave the film unable to level, giving dry spray, orange peel and pinholes. Solvents that evaporate too slowly leave the film soft, prone to sagging on vertical surfaces and to solvent entrapment under the next coat.
Beyond that, the wrong solvent chemistry may not properly dissolve the binder at all, disturbing cure and reducing chemical resistance and adhesion in ways that only appear months later in service.
Match to system and to temperature
Epoxy and polyurethane thinners are not interchangeable, and manufacturers often publish separate summer and winter grades. In Dubai summer conditions a slower evaporating thinner helps the film flow out before the solvent leaves, exactly the opposite of what a cold climate needs. Using a fast winter blend at 42 degrees ambient produces the dry spray that crews then try to fix with more thinner, compounding the problem.
Respect the maximum addition
Data sheets state a maximum thinner percentage, typically a modest single figure by volume. Exceeding it reduces volume solids, so the applied film is thinner than intended at the same wet thickness, causing the coverage calculations in our estimating guide to fall apart. Over thinning also raises VOC content, which matters where emissions are regulated or reported.
Cleaning is a separate question
Equipment cleaning solvent need not be the application thinner, and using expensive matched thinner for flushing wastes money. Keep them separate and labelled, and never return used cleaning solvent to the application thinner container.
Practical control
Keep the correct thinner in stock alongside the coating, in labelled containers, and measure additions rather than pouring by eye. If spray quality is poor, check gun setup, tip size, pressure and temperature before reaching for more solvent.
MENA Industrial Supplies stocks epoxy, polyurethane and general purpose thinners matched to major coating systems, in 4 litre, 20 litre and 200 litre packs.