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.

Paint Troubleshooting – Sagging

Sagging happens when the paint is not viscous enough to hold itself together before it dries up. This defect is particularly common in liquid systems. If this defect is encountered, you may check the following list below where you might have gone wrong with your paint preparation or application.

Preparation:

1. Use only the correct diluents (either solvents, or water) as recommended by the paint manufacturer. Particularly speaking about OEM paint, I suggest that you only use the thinners that came with the paint. Should you use an alternative, seek proper advise to avoid paint defects.

2. Use the proper dilution ratio. Do not be any means overdilute paint. It will not only throw you off the viscosity recommended for spraying, the overdilution is also undoable. As good practice, add only half of what is the recommended by your supplier and check the viscosity in between dilutions. That way you will not overdilute your paint. Dilution ratios are a good guide to help you mix paint. Extra care should be observed if you are using a new paint system or color for the first time. Remember, we add diluents to paint to obtain the proper viscosity for application.

3. Check your expiry dates. Paints while kept unopened in original containers are good for a year, kept in controlled environments can be kept for two years. However, there are paint formulas that are stable enough that are still good even after several years provided that they are sheared with a mixer.

4. Do not overmix paint. Some paint thin (lose viscosity) while some thicken (become more viscous) after mixing with a powermixer. Be sure to follow the manufacturer’s recommendation for mixing time.

Application

1. Humidity. To achieve the best appearance from your paint (particularly for OEM paint), paint application should be performed in a paint booth, with exhaust and blowers to control not only the temperature but the humidity inside the booth. If there is too much humidity in the air, the diluent/s in the paint might not flash-off as expected leaving the paint wet for a longer time allowing more opportunity for the paint to sag. On the same note, if the flash-off is too fast, you might get solvent popping for bake-type paints (for cars, drums, parts, etc).

2. Wet paint thickness. Check your paint/air flowrates in the spray gun. You may also consider modifying your spray pattern if the paint applied on a part is uneven. If the paint is applied too thickly, the paint film will have a higher tendency to sag. The same goes for brushed or roller applied paint.

Paint Systems - Application

Paint can be applied in numerous ways. It can be applied by brush or roller, by spray guns, by dipping, by roller applicators, by applying electricity, or by your fingers! There are certain considerations to determine the most effective way to be employed in choosing the best technology available.

1. Cost
How much are you willing to spend in putting up your facilities? This will also dictate operating expenses that you will be incurring. This will also dictate the type of paint you will be using. Depending on the throughput that will go through the facilities, it is best to consider not only the present requirement but, also future expansions of the line as well as downgrades. Electrodeposition or ecoat lines normally require bigger capital expenditures compared with powder coating lines, but it is cheaper to run an electrodeposition line compared with a powdercoating line. Ecoat baths can be used indefinitely as long as the paint bath quality is maintained, as long as there are no contamination issues encountered. Powdercoat can only be used twice, then discarded.

2. Quality

The technology to be used can also be dictated by the quality of the appearance finish of the surface to be painted. Gloss, roughness, color are some properties that may be of great significance. If the thickness of the paint film to be applied is an issue, if thickness should be between 20~25 microns, a brush application is not applicable, similarly, if thickness requirement for a paint film is over 100 microns thick, electrodeposition is not an option.

3. Parts to be painted

The size of the parts or surfaces to be painted should also be considered. Some parts sizes and shapes are not possible to be painted by powdercoating. A Faraday cage phenomenon is a very big consideration. In OEM painting, plastic or metal parts would dictate the paint technologies applicable.

Other factors may eventually come up as the painting process is being conceived. Some factors may weigh more than others and a thorough investigation and further study should be performed to assimilate most, if not all, all these concerns.

About this blog

I am going to start on this blog to help the many people who are involved in the paints and coatings industry.