paint dillution

[SteamGene]

I'm about to fire up a new paint gun for weathering, the first victim being coaling and sanding towers.   I plan on using a water based paint, probably mixing grimy and oily black 50:50.  What would be a good dillution to represent the amount of smoke that would gather by 1957?  The large towers are concrete, erected in the 20s and the smaller ones are wood, probably from late 19th, early 20th century.  I'll proably do the tunnel portals, too.
Gene

[grumpy]

your dilution maybe determined more by the viscosity that the spray gun will laydown smoothly and evenly than by the colour you wish to achive'The only way to determine the dilution will be by experimentation. Most generic paints will work at 20%.
Don

[SteamGene]

I know that.  But I want it dilluted for weathering, not coverage so I know it's more than 20%.  I believe it's more than 50%.
Gene

[r.cprmier]

Gene;
Take some time to experiment with it.  I have found that, although sound proceedure, the manufacturers will give average guidelines-except for care of product.  Follow their instructions to the letter.

In mt airbrushing experience, I have found that trial and error-and keeping track of what I did-is pretty helpful, so take some time, spray some patterns using various mixes, ratios, etc; until you find what grabs you.

Rich

[SteamGene]

Rich,
Okay.  I've never used an air brush for weathering before.  I'll try.
Gene

[ebtnut]

For weathering purposes, you generally want the paint to be about twice as thin as it would be for spraying full color.  You want to be able to build up the weathering, and sometimes feather it too.  Gene is right--test spray on something else to get a feel for how the color goes on.  You will likely want to take several passes and see what the effect it between each pass.  Also pay some attention to how weathering occurs.  Most equipment picks up more dirt and grime at track level.  Other stuff collects on the roofs.  In addition, you will often see dirt stripes on the ends of rolling stock where the wheels of the coupled cars splash dirty water up on the other car end.  And it depends on what your modeling era is.  Steam era weathering would inlcude more grey/black from soot and cinders, especially on horizontal surfaces. 

[WoundedBear]

And then there is the whole theory of "scale colour". This meaning, the lightening of colors to simulate viewing through the haze of the atmosphere.

As a rule of thumb, the theory says to add white, or other colors to the base color to soften them. The smaller the scale the lighter the shades. If you do a search on some IPMS sites they have a few pages explaining it.

The same theory applies with scale lighting and model building. Here's a couple of links to look at.

http://www.j-aircraft.com/faq/scale_effect_of_paint.htm

http://www.paintassistant.com/scalelighting.html

Sid

[SteamGene]

The first thing I'll work on are the coaling, sanding, and watering towers followed by the tunnel portals.  I've weathered tunnel portals before with a brush and dilluted greasy black paint.  The ones I've done look fairly good, but I want to try the new air brush anyway. 
Gene

[WoundedBear]

You will soon discover that the airbrush is the ideal way to weather, and is so controllable that you will be reaching for it time and time again.

Sid