It's true that the metallics are trickier to spray than the solid colors. Uneven color and gloss, and the dreaded "tiger stripes" are problems that occur more often with metallics than with solids.
SPRAYING METALLICS
The goal is to lay down the paint (and the suspended metal flakes) as evenly as possible. Many things can prevent this.
Putting to much paint on in one pass can cause uneven color. And if the gun is held too close unevenness will happen. Also, a tip too large will cause the same problem. So will moving way too slow, or not having enough overlap on passes.
FULL TRIGGER
Matthews recommends two medium heavy passes with a flash time in between. This means full trigger for both passes, not half trigger. Many autobody painters are in the habit of applying a light "tack" coat followed by a heavy coat. Matthews is not engineered to be applied this way.
And the purpose of the flash time in between is to allow the first coat to set up somewhat before you spray the final coat. This is what allows the film thickness to build up properly. Matthews achieves its best durability when the correct film thickness is reached. At the Matthews training school, we were all given a small tool that measures wet film thickness and shown how to use it. We were also taught how to calculate the dry film thickness using the wet film thickness.
How long should the flash time be? Actually, it can vary depending on the weather and your reducer mix. If you touch the paint and it comes off on your glove, you haven't waited long enough.
When spraying, the first pass should pretty much cover the substrate without thin spots or obvious stripes. 50% overlap is a good rule, like
euippaint said, though you can overlap more for metallics. Spray the second coat at right angles to the first. A good rule of thumb for gun distance is 8-10 inches. Spray parallel to the surface, without "fanning" the gun, as fanning causes uneven paint distribution. Generally, you want to spray so that each subsequent pass buries the overspray of the previous pass. On a panel that is flat on saw horses, start at the edge closest to you and walk forward. This is not a critical rule, and there are many times when overspray will unavoidably land on previously painted areas. But burying your overspray whenever possible is a "best practice," a good habit to develop. This means that on a vertical surface, start at the top.
Obviously, if you are painting a 4x10 panel lying flat, you can't reach far enough to make passes across the entire four feet of the panel's width. You will walk down one side, spraying a 24 or 30-inch swath and then walk over to the other side and do the same thing. When spraying the second side, the overspray from the ends of your passes will land on the side you painted first. This overspray should melt into the first side without a trace. If it doesn't, your mix is probably too fast. Use a slower reducer, or mix a slower reducer with the one you're using, or add retarder.
A FINAL LIGHTER COAT SOMETIMES WORKS
Spraying the second pass at right angles to the first pass, "crosshatching," can go a long way to eliminating unevenness in metallics. If I'm having a problem with unevenness, I will sometimes spray a third coat right after the second coat. I will back the gun away from the panel a little more than usual for this pass, and move a little slower than usual so that it goes on a little lighter. I let this finer coat settle down onto the wet second coat. I have achieved incredibly even metallics this way. But it only works if your paint is mixed on the slow side. I often add a little retarder when painting metallics for just this reason.
If you spray Matthews metallics be aware that some of the colors should be clear coated. If you are adding converter to your paint, you will be clear coating anyway. Matthews converter turns any Matthews color into a "base" coat that dries quicker than normal with not much gloss, which then must be cleared over for durability. Matthews converter creates, essentially, a base coat/clear coat system.
What is the advantage of converting colors to base coats? It allows you to spray and mask several colors in one day and then spray clear as the final coat at the end of the day. Theoretically, you are able to spray two, three or four colors, with taping in between, in one day (depending on conditions). Being able to install a two or three color job the next day, rather than waiting a day for each color to dry enough for taping, is obviously advantageous.
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In my post above I linked to the Matthews Training Manual. In the table of contents is listed a Troubleshooting Section which isn't there. I didn't realize until today that this online document has only half the manual.
The troubleshooting section that is included in my printed manual is on the website but in another section under a subheading entitled Company Literature.
Here is the link to the troubleshooting section:
http://www.matthewspaint.com/getmed...d5b9e28825/mpc-troubleshooting-guide.pdf.aspx
Brad
