LAST EDITED ON Sep-19-01 AT 08:43 PM (CDT)
Xain,
Couple of related threads to check out here on the SSW forum. Here are the addresses. Hope is these may enlighten, rather then confuse.
1)
http://www.clstunt.com/htdocs/dcforum/DCForumID1/701.html#
2)
http://www.clstunt.com/htdocs/dcforum/DCForumID1/768.html#
My understanding of the "Blush" problem is this. Depending on the amount of moisture in the air and the air temp, somewhat dictates how much trouble you will have with "blush".
Different thinners have different rates at which they evaporate at. If the thinner is "fast" the evaporation rate is high, and the drying time low. If the thinner is slow the evaporation rate is slow, so drying times gets longer.
During the process of evaporation a change of state is taking place for the thinner, it's changing from a liquid to a gas or vapor. This process takes energy in the form of heat. As the thinner is heated (by the ambient air temp) and changes from liquid to a gas/vapor, it's pressure also drops.
Hand in hand with the pressure drop is an auto re-fridge phenomena, ...a cooling thing happens. That is to say that as the thinner flashes off the temperature at the surface where the change of state, liquid to gas/vapor, is taking place also drops. At the site of this temperature drop is where the blush takes place.
If the rate of vaporization is high enough, and a resulting low temp at the surface is created, the moisture resident in the ambient air drops out of it's gaseous state and is trapped in the surface of your dope finish. In other words; the temp change at the surface of your model is low enough to condense out the moisture that is already present in the surrounding air. This is what shows up as "blush".
Ever notice how cool your hand gets after spilling model fuel on it? Same vapor/cooling thing happening here as there.
To combat this tendency to blush you needed to slow down the rate at which the thinner flashes off to vapor. A slower thinner is required. For the automotive industry they have many different thinner/reducer mixes to pick from for the temp range they are spraying in. We do not have this option, but we do have "retarder" in most cases. Sig and Brodak (my dope of choice) both have retarder available to help control blush.
I've used it both to control blush, and as a means to help Brodak dope flow a little better.
I've found that when using a full size HVLP gravity feed automotive spray gun that Brodak needs to be thinned better then 60 to 70% and retarder is needed in as much as 20% of the total thinner make-up. So my spray mix totals would read something like this: dope-40%, thinner-40%, retarder-20%.
With this I get both a slower thinner(reducing the blush tendency) and a resulting slower drying dope (helping the dope to flow out better)giving a smoother surface having less orange peel.
These numbers are not carved in stone, but it's what has worked for me during the very hot humid summer days I've been spraying in this summer. As advised by Windy Urtnowski do a test shot of the mix you would like to try spraying on a small area. If the test is good go for it, ..if not re-mix the ratio's and try again.
More theory: The hotter the day the more energy available for thinner flash off.
The higher the humidity, the more moisture in the air for possible blush production.
Also: Your air supply is a source for moisture in the air stream pouring over the model, ..ensure it is as dry as possible.