Sprayed undercoat surface powdery

[Dengue]

Hello, I am using a Dulux Duramax Undercoat spray cans on a workpiece, prior to applying a topcoat of Dulux Duramax Spray enamel.

I have applied the undercoat as three thin coats, with the can constantly moving fairly quickly, and about 6" from the workpiece. There were no runs.

The air temperature is close to 30 degC today.

The problem is, that after the recommended 2 hours curing time has elapsed after each coat I can run my finger along the workpiece and this white powder is left on my fingers.

I can sand the workpiece back with 800 grit, and it is nice and smooth.

Can anyone please comment on the powder, and also , after sanding smooth, should I proceed with the top coats?

[China]

Just dry overspray, don't think you should have any probs with topcoat

[Mobyturns]

With a spray gun the operator has control over spray pattern, solvent ratio, atomization etc however with a spray can you do not have those options. The dry powder is the result of solvent evaporating before the solvent / solids mix hits the target surface, i.e. drying early. Ideally the mix should land as a "wet" surface that permits the solvent to evaporate at the ideal rate to promote controlled drying and a level surface.

Some dry powdery over spray is normal as spray painting is not an efficient process. Equipment choice and procedures do improve efficiency. All you can do is make sure the spray cans contents are well mixed (shaken), ensure the can's nozzle is clear, trial various spray distances, and avoid spraying on dry hot days that promote fast solvent evaporation.

Application thickness also determines how fast or slow the solids dry. Light coats on hot days increase the rate of solvent evaporation and interfere with the operators ability to produce a uniform thickness film coating. Two thicker wet coats may be more ideal than 3 or 4 light dry coats. Heavy wet coats are slow to dry and introduce other problems like "runs" etc. The solvent on subsequent coats may also key - partially dissolve, the previous coat/s i.e. nitro cellulose lacquer.

If the surface is relatively level and a uniform thickness it should be OK. Most spray finishes require additional leveling to obtain a high quality appearance.

[Dengue]

Many thanks to China and Moby. Looks like you are both correct. Firstly, I was spraying two thick boards sitting parallel 6” apart, so there definitely was overspray as I was painting the sides of the boards too. Also the instructions on the can says not to spray when 30degC or more. It was just above that yesterday at times, so there were times when the proper mix of solvent and paint were not hitting the workpieces.

I suspect the other problem is humidity. The aerosol coming out of the spray can is quite cold, and condensation of the very high humidity on the droplets would not do the finish any good.

SO.....armed with the knowledge provided by China and Moby, today I sanded the undercoated boards to 1200 grit. I then waited until 8am when the humidity had dropped below 80% (was 95% 6-7am) and the temperature had not yet reached 30 degC, then sat only one board on the turntable and applied the first of many top coats. I also moved the can a bit closer and a bit slower to give a wet finish.
The end result was 1000% better than yesterday. I will not have to give the finish a light sand with the 1200 grit before the 2nd coat.
Thanks for all your help,