Paint Troubleshooting - Soft film

The best time probably to paint your house is in the summer time where there is enough sunshine during the day and less humidity to speed up the drying process.

The best time of the day to repaint your car is in the afternoon. Try repainting your car in an uncontrolled environment, no spray booth, and by lunch time you'll be seeing small crater like holes on the paint film. Not only will you need to repaint, you might also need to strip the paint off back to bare metal.

On to my topic. How can you tell if your paint is too soft than it should be. There are several tests you can perform but I will discuss here the simplest ones you can do without the use of complicated equipment.

Nail Mar

This is done by simply denting an inconspicuous portion of the painted surface with your fingernail. Normally the thumb is used to do this. If the scratched surface is not removed after wiping then, the paint is too soft.

Coin Mar

Although not commonly used in other industries excepted the coil coating industry, this test is commonly used for its simplicity. Although the force applied is highly contestable and often subject to argument. The coin used for the test is normally agreed upon by the consenting parties, if the end user of the paint (customer) uses a quarter to test, the supplier should also use a quarter for lab tests.

There are also more accurate means to test for hardness, there is the use of pencils and impact testers. Pencil brands such as Staedtler and Faber Castell are common choices. Impact testers are also available from BYK-Gardner, Sheen and Elcometer. There are several brands to choose from. There are also many tests that you can follow. Some these iclude ASTM, DIN and JIS, depending on you and your customer or you and your supplier.

So what causes a soft film?

1. Paint is not dry. For air dried paint, allowing more time to dry would be enough to solve the problem.

2. Dilution ratio was not followed. Some paint systems are 3K or three component, meaning the paint is separate from the hardener or catalyst and the solvent. If the hardener or catalyst was not followed and either too much or too little or the wrong one was added, the dried paint film may be too soft for the control parameters. Double-baking would not solve this problem because double-baking usually causes color to be darker compared to a standard. Care should be taken when repainting because you might have issues with adhesion, telegraphing and shrinking.

3. The baking curve was not followed. Normally, a baking curve or window is supplied by your paint supplier, this would give you flexibility with the oven temperature, and baking time. A lower baking temperature would require a longer baking time and a higher baking temperature would require a shorter baking time. this would all be dictated by the baking window/curve.