Friday, November 22, 2013

Metallic mil-spec Topcoat Spraying

Metallic Mil-Sec Coatings


Don’t Spray Like This

In spraying most mil-spec coatings topcoat colors our painters get into a nice routine of spraying a certain way that brings us satisfactory results.  But there are a couple of metallic colors in the Federal Standard 595 color deck that you can’t spray the usual way at least without getting out-of-spec color appearance. 

There are 650 colors currently in the 595 color deck.  But 4 of those colors are metallics:  17043 gold, 17178 silver, 27043 semi-gloss gold, 37200 flat silver.  And one of those colors, 17178, is used fairly often for aerospace coatings applications.   If you spray 17178 like you usually spray Mil-PRF-85285 solid colors with 2 wet coats, you will probably think you have received a bad batch of paint.  This color in both Mil-PRF-85285 and in Mil-PRF-22750 has come up in more complaints than any other color.  And upon further investigation the problem usually comes from the improper spray techniques.  You can’t spray metallic colors the same way as you spray the rest of the solid colors in the 595 deck. 

Instead Spray Like This

Metallics must be spayed in 3 deliberately light passes with the spray gun.    Painters with automotive paint experience have an easier time with these metallic colors because they have had experience with spraying metallic paints during their automotive careers. 

Other keys to spraying metallics:

Proper mixing: Follow the recommended mix ratio to the letter.  If allowed use some thinner as this will help to achieve better atomization.

Low fluid delivery: Lower your fluid delivery to 1/2 to ¾ that you used for solid colors.  Look for fine atomization to reduce mottling & to bring out the metallic.

Multiple Passes: Start with light tack coats, cross pattern (1.0-1.5 mils Wet)

Allow 1-5 minutes flash.  Build film slowly with light coats, again cross coating.

Do not allow film to get too wet. (3.0-6.0 mils wet).  To bring out bright metallic fog the last coat from minimum 1 foot distance.  Total DFT should be 1.5-2.0 mils for optimum gloss and color consistency.

Consistent basecoat or Primer color: Always do original color match on the same substrate you’re your customer is going to use.  Gray primers impart darkness and blueness, buff primer brings out yellow color.

Increased the distance from object to be painted:  Painting too close with metallics will cause mottling, edge pull, darker color.  Keep spray gun further back.

Agitate metallic paints often: Before mixing paint and catalyst, agitate metallic base component well.

After paint is reduced for spray, swirl cup gun gently before each coat.  Or for large volume applications an agitated pressure pot is suggested.