How to Thin Enamel Paint for Spraying: What Data Sheets Say

Enamel is not one paint, so there is no single thinning ratio. Rust-Oleum’s data sheets for its oil-based enamels allow up to 15% acetone or its own branded thinner for air spray and warn against lacquer thinner, while Sherwin-Williams lists no reduction needed for its waterbased trim enamel. This guide shows what those sheets say, how to test a batch, and where the safety limits sit.

Check the Data Sheet Before You Pick a Thinner

The common advice is “mineral spirits for oil enamel, water for waterbased enamel, 10 to 15 percent.” The manufacturer sheets are more specific, and in one case they disagree with it. Rust-Oleum’s Stops Rust Enamel Brush is an oil-modified alkyd with mineral spirits as its solvent, yet its data sheet names acetone as the thinner and keeps mineral spirits for cleanup. The table below pulls the spray lines from four sheets, so you can see how much the answer changes from product to product.

Product (data sheet)	Thinner named	Air-atomized spray	Airless spray
Rust-Oleum Stops Rust Enamel Brush (form GDH-705)	Acetone	Up to 15%	Thinning not normally required
Rust-Oleum Farm & Implement Enamels (form GDH-959)	Acetone	Up to 15% by volume	Normally not required, up to 5% if needed
Rust-Oleum ROCAlkyd Direct to Metal 7400 (form EJ-67)	Rust-Oleum thinner, item 333402	Up to 15% by volume	Normally not required, up to 5% if needed
Sherwin-Williams Emerald Urethane Trim Enamel, waterbased	None, no reduction needed	HVLP listed with no reduction	Tip .013-.017 inch at 2000 psi, no reduction listed

Sources: Rust-Oleum Stops Rust Enamel Brush technical data, Rust-Oleum ROCAlkyd 7400 technical data and Sherwin-Williams Emerald Urethane Trim Enamel product data sheet. The Farm and Implement figures come from Rust-Oleum’s SPC-66 sheet. Sheets get revised, so check the current one for the exact can in your hands.

Read the pattern in that table. The three oil-based sheets cap air-spray thinning at 15%, and all of them treat airless spraying as a no-thinning or 5% job. The 15% is a ceiling printed by the maker, not a starting point. If you came here after our Rust-Oleum spray gun guide, this page is the broader version for any enamel brand, and it is built around those data sheets rather than a single brand’s range.

Oil-Based Enamel: Acetone, Branded Thinner or Mineral Spirits?

Oil-based (alkyd) enamel is a solvent-borne paint, so only a solvent thins it. Which solvent is the part people get wrong. On the Rust-Oleum sheets above, the thinner is acetone for two product lines and a Rust-Oleum branded thinner for the third. Mineral spirits shows up on those sheets as the cleanup solvent, and as the solvent already inside the Stops Rust brush enamel.

Both the Stops Rust and the Farm and Implement sheets add a warning in plain words: do not thin with gasoline, lacquer thinner or turpentine. That matters because lacquer thinner is one of the first things people grab, and Bing searches on this very question (“what happens if you thin oil base enamel with lacquer thinner”) show readers asking it. The honest answer from these sheets is that it is not on the approved list. If your can names a different thinner, the can wins over any website, including this one.

“Do not thin with gasoline, lacquer thinner, turpentine, etc.”

The sheet for Stops Rust Enamel Brush also gives a flammability number worth knowing before you open a container near a heater or a pilot light.

Flash point 104°F (40°C) for Rust-Oleum Stops Rust Enamel Brush, a temperature a hot garage can reach in summer. Source: Rust-Oleum technical data GDH-705

Acetone is a separate solvent with its own safety data sheet, so read that sheet before you buy it for thinning. Keep ignition sources away from any thinning or spraying area, and do not use the spray gun near an open flame.

Volume math for the 15% and 5% limits

The sheets give percentages by volume, which people then eyeball. Convert them once and measure with a marked mixing cup. These are straight arithmetic on the sheet limits, not recommended starting amounts.

Paint volume	15% (air spray ceiling)	5% (airless, if needed)
1 pint (16 fl oz)	2.4 fl oz	0.8 fl oz
1 quart (32 fl oz)	4.8 fl oz	1.6 fl oz
1 gallon (128 fl oz)	19.2 fl oz	6.4 fl oz

Thin from the other direction. Add a small amount, stir for a full minute, test a pass on scrap, then add more only if the paint still spits or orange-peels. Because the sheet says to stir thoroughly before using, this also gets any settled pigment back into the batch before you judge it. If a batch is still fine at 5%, stop there.

Waterbased Enamel: Often No Thinning At All

Waterbased enamel is the part of this topic where the usual rule of thumb is least backed up. Many guides give a flat 5 to 10 percent water figure. The Sherwin-Williams sheet for Emerald Urethane Trim Enamel says “no reduction needed” and lists an HVLP setup with no reduction: a Graco HVLP 9.5 with a number 3 or 4 needle and cap, two-thirds air pressure, and a number 5 material setting. Its airless line is 2000 psi with a .013 to .017 inch tip.

That does not mean no waterbased enamel ever needs water. Other brands and other lines publish their own numbers, and a very thick can or a cold shop can change how it atomizes. It means the safe order is the data sheet first, then a test spray, then a small water addition only if the sheet allows one. The Sherwin-Williams sheet also lists application above 50°F and a VOC level under 50 grams per liter.

One distinction from reader searches: a lot of people ask whether a “urethane enamel” can be thinned with water. Urethane-modified alkyd enamels exist in both families. The Sherwin-Williams Emerald line above is waterbased, so its sheet names no thinner at all, while an oil-based urethane alkyd is a solvent paint. The label tells you which one you have. For the latex side of this question, see our guide to thinning latex paint with water.

Test the Batch With a Viscosity Cup

None of the four data sheets above gives a viscosity-cup time. They give percentages. That leaves a real gap, because two cans of the same enamel can flow differently depending on temperature and age, and a measuring cup cannot tell you that. A flow cup turns the gap into something you can repeat.

The method is simple. You dip the cup until it is full, lift it out, and time how long it takes to drain completely. The number only means something compared with the same cup, so treat it as your own log rather than a universal target. Spray a test pass, and when the finish lays out smooth with no spitting, write down the seconds next to the percentage you used.

1. Stir: mix the enamel thoroughly so settled pigment is back in suspension.
2. Strain: pour it through a paint strainer into a clean container.
3. Measure: add the thinner by volume, starting well under the sheet limit, and stir again.
4. Time: fill the cup, lift it, and time the drain to a full stop.
5. Spray a test pass: use scrap that matches your project and look for spitting, runs or a rough orange-peel texture.
6. Log it: write down the percentage and the seconds for the next batch.

The next batch of the same paint in the same temperature range can then be thinned to that logged number, not guessed. If you also work with clear finishes, our guides on thinning polyurethane with paint thinner and spray guns for polyurethane cover the same measure-and-test habit for those products.

Spray Setup Numbers From the Sheets

Thinning is only one half of atomization, and the sheets also give gun-side numbers. For ROCAlkyd 7400, Rust-Oleum’s listing for an HVLP gun is a fluid tip of 0.043 to 0.070 inch at 8 to 14 ounces per minute, with a note of 10 psi maximum at the tip. For airless, the same sheet lists 1,600 to 2,400 psi, and tip sizes of 0.013 to 0.017 inch for high gloss and 0.013 to 0.019 inch for the other sheens.

Conditions matter as much as the gun. The Stops Rust brush enamel sheet says to apply between 50°F and 90°F with humidity below 85 percent, and the ROCAlkyd sheet allows 32°F to 100°F with the surface at least 5°F above the dew point. Most hobby guns are not rated against these figures, so check your gun’s own manual for its fluid tip and pressure range. If you are still deciding between a brush and a gun for a job, brushing vs spraying paint lays out the tradeoffs.

Fumes, Flammability and Respirators

This is the section to slow down on. Solvent thinners and the enamels they go into give off vapors that are both flammable and harmful to breathe. The Stops Rust brush enamel has the 104°F flash point shown above, and Rust-Oleum’s aerosol enamel sheet says to use it outdoors or in a well-ventilated area and not near open flame. The waterbased Sherwin-Williams sheet still says to use only with adequate ventilation and to wear NIOSH-approved respiratory protection if you get eye watering, headaches or dizziness.

• Spray outdoors or with strong cross ventilation, and keep pilot lights, heaters and sparks away from the work area.
• A paper dust mask does not filter solvent vapor. The usual choice for solvent spraying is a NIOSH-approved respirator with organic vapor cartridges, but the exact respirator for your product is named in its safety data sheet.
• Wear nitrile gloves and eye protection when mixing, and keep containers closed when not in use, as the Sherwin-Williams sheet says.
• If you scrape or sand old paint first, the sheets warn that it may release lead dust, and call for a NIOSH-approved respirator and HEPA cleanup.

Left-over thinner and rags need their own care. Our note on paint thinner and the drain covers why it should not be poured out, and your local hazardous waste rules decide what to do with soaked rags.

Common Mistakes When Thinning Enamel

• Using lacquer thinner by habit: two Rust-Oleum sheets list it as a thinner not to use on those enamels.
• Treating 15% as the target: it is a ceiling for air spray, and airless usually needs none.
• Thinning waterbased enamel because a website said 10%: the Sherwin-Williams sheet read for this guide lists no reduction.
• Skipping the stir: the Rust-Oleum sheet says to stir thoroughly before using to re-disperse settled pigment.
• Spraying in the cold: the sheets give minimum temperatures of 50°F or 32°F depending on the product.
• Never logging the cup time: without a record, every batch is a fresh guess.

If you are unsure which paint belongs on a wood project in the first place, our guide to what kind of paint to use on wood sorts primers, latex and oil. For clear coats, oil vs water-based polyurethane and applying polyurethane without brush marks cover the finish-side questions this page does not.

Frequently Asked Questions About Thinning Enamel Paint

Can you thin enamel paint with water?

Only if it is a waterbased enamel, and many do not need it. Sherwin-Williams lists no reduction needed for its waterbased Emerald Urethane Trim Enamel, including for HVLP spraying. Oil-based enamels use a solvent instead, so water is the wrong choice for them.

What thinner do you use for enamel paint?

Use whatever the can or data sheet names. Rust-Oleum’s Stops Rust enamel brush sheet names acetone for thinning and mineral spirits for cleanup, and its ROCAlkyd sheet names its own branded thinner. If your can names nothing, check the maker’s data sheet before buying a thinner.

Can you thin oil-based enamel with lacquer thinner?

Not on the sheets read for this guide. Rust-Oleum’s Stops Rust and Farm and Implement enamel sheets both say not to thin with lacquer thinner, gasoline or turpentine. Lacquer thinner is a different product from the acetone those sheets name.

How much should you thin enamel for a spray gun?

Use the figure on your own product’s data sheet. The Rust-Oleum oil-based enamel sheets read for this guide allow up to 15% by volume for air spray, and normally no thinning for airless. Waterbased trim enamel from Sherwin-Williams lists none. Add small amounts and test spray on scrap.

Can urethane enamel be thinned with water?

It depends on the type. Sherwin-Williams Emerald Urethane Trim Enamel is waterbased and needs no reduction per its data sheet. An oil-based urethane alkyd is a solvent product, so water does not belong in it. Read the label to see which family you have.

Conclusion

Thinning enamel for spraying starts with the data sheet, not a rule of thumb. The Rust-Oleum oil-based sheets cap air-spray thinning at 15% with acetone or the branded thinner and say not to use lacquer thinner, and the Sherwin-Williams waterbased trim enamel lists no reduction. Measure by volume, add little, test on scrap, and log your cup time so the next batch matches. Keep solvents away from flame, and read the safety data sheet before you open the can.