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Delay time between chromate conversion coating of aluminum and priming/painting

Maximum time between chem-film and priming

Q. I just inherited an old program. Two aluminum parts, one is a casting alloy A357.0 T6 grade B, class 10 and the other is 7075-T7351. Both are chem filmed per MIL-C-5541 and subsequently primed and painted. The drawing requirements state that the prime must be applied within 3 - 10 days after the chem film. Is there an actual time requirement stated anywhere, what's the reason for it, and if so where can I find it?

Paul [name deleted by Editor due to age of posting]
an aerospace company - Pine Brook, New Jersey, USA

A. One of the MIL specs had a requirement for 24 hour delay between chem film and paint. I think that the rationale is to let the "gel" properly set before paint.

James Watts
- Navarre, Florida

Minimum time between chem-film and priming

Q. We are trying to determine a minimum amount of time that we need to wait after we chem film aluminum until we can apply primer and paint. We are currently waiting 24 hours after chem film to prime, but cannot find it specified anywhere that I know of. I would like to shorten that time if possible to compress cycle time. Can anyone tell me what the minimum time is?

Kelly Waters
program manager - machine shop - Crestview, Florida, USA
January 13, 2011

A. Kelly,

I don't think there are any hard and fast rules about how long you have to wait to paint on conversion coated surfaces, it really depends how dry the surface is before you paint. I have seen as little as four hours and as much as 48 hours documented as the delay time.

In the end it is what you feel comfortable with. Currently we force dry our Alodine at 55 °C for about an hour in an air circulating oven before shipping to the paint department. Parts are usually painted within 24 hours, but may be painted as soon as 8 hours after coating.

I can only suggest that you assess your conversion coating and painting processes and see whether you can paint early. You may want to trial shorter times between the two and check that you maintain paint adhesion.

Brian Terry
Aerospace - Yeovil, Somerset, UK
January 14, 2011

A. We force dry for 20 min after a light dust coat then apply a primer, wait another 20 min and top coat, repeat another top coat (two top coats total), it works for us and haven't had any issues?

Peter Crawshaw
- Featherston. Wellington 5710. New Zealand.
October 22, 2023

Q. I have to apply MIL-PRF-23377 [on DLA] primer on MIL-DTL-5541 [on DLA] chem filmed aluminum alloy die casted and machined parts.

Is there a time limit to apply primer for good adhesion?

raj shah
- santa ana, California usa
August 12, 2014

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