Thinner ratio is one of those specs everyone asks for a single right answer to, and there genuinely isn't one. The correct ratio depends on the paint system, the spray gun, ambient temperature, and the finish you're chasing. What follows is the reasoning behind the number, not a substitute for the product's own data sheet — always check that first.
Why ratio matters
Thinner reduces the paint's viscosity so it atomises correctly through the gun. Too thick, and the gun struggles to break the paint into a fine, even mist — you get orange peel, poor flow-out and visible spray pattern lines. Too thin, and you get runs, sags, reduced film build, and the paint can dry too fast in the air before it reaches the panel (particularly with cellulose systems).
Common ratio ranges
These are typical starting points — always confirm against the specific product's technical data sheet, since formulations vary between manufacturers:
- Cellulose paints — often thinned more heavily, sometimes 1:1 or higher, and adjusted further by temperature. Our cellulose clear thinner is formulated specifically for this system.
- 2K basecoats and primers — typically a lighter thinning ratio, commonly in the range of 10–20% thinner by volume, though this varies significantly by product.
- 2K lacquers — usually mixed with hardener first at the specified ratio, then thinned separately if the data sheet calls for it — don't skip straight to thinning without checking the hardener ratio first.
Our Standard Thinner 400 is a general-purpose automotive-grade thinner suited to a range of systems — but "general purpose" still means checking the paint manufacturer's own guidance for the specific ratio.
Temperature changes the answer
Thinner speed (fast, medium, slow) needs to match ambient temperature, and this interacts with ratio:
- Cold conditions — paint stays wet longer, increasing run and sag risk. A faster thinner and sometimes a slightly reduced ratio helps.
- Hot conditions — solvents flash off quickly, risking dry spray and poor flow-out before the paint levels. A slower thinner, and sometimes a touch more of it, keeps the paint workable long enough to lay down properly.
A booth running warm on a summer afternoon and the same booth on a cold winter morning are not the same job, even with identical paint.
Gun setup matters as much as ratio
Nozzle size, air pressure and fan pattern all interact with viscosity. A paint mixed correctly for a 1.3mm nozzle will behave differently through a 1.8mm nozzle at the same pressure. If you change guns or nozzle size, re-check your mix rather than assuming the same ratio still applies.
Practical mixing tips
- Mix by volume using a proper mixing stick or graduated cup — measuring cups matched to common ratios remove the guesswork.
- Always mix hardener first (on 2K products) at the specified ratio before adding thinner.
- Test spray on scrap card before committing to the panel, especially after changing ratio, gun or conditions.
- Mix only what you'll use within the pot life — don't stockpile mixed paint.
See the full paint and bodyshop range for thinners, lacquers and primers, or trade bundles for bulk quantities.
