Charles - It all depends on the brand / compatibility of the dope you are using with standard available thinners. The dope pretty much has one speed, and the thinner determines the set. If the dope you use can be thinned with auto thinners, then get some acrylic lacquer slow rated thinner (not the cheap stuff for cleaning brushes)

Retarder is probably made mostly of the same chemical that makes slow thinners slow, but concentrated. You have to be very careful using it, as it's easy to go overboard and you can melt a plastic canopy with the stuff, as well as your previous layers of dope come bubbling up. I'd personally rather use a slow rated thinner that way it is essentially pre-mixed with retarder to the correct amount for you.

In sunny Florida, I like slow thinners in the summer, medium in the winter, and almost never fast. This is because the ambients do effect the cure time. A slow thinner used on a cold winter day could be disastrous, where the same thinner would work fine on a sunny summer day. One might think slower is always better, since it will have more time to level out and less chance at trapping moisture and blush. There is such thing as too much of a good thing here, because if it's too slow, then it will have time to eat/dissolve down into previous layers (very bad when shooting trim colors or clear coats over final paint scheme) especially if you are the kind who likes to gun on a heavey wet coat rather than build up several lite coats.

So, like all things in stunt, it's an acquired talent to get a feel for when to use what. It's like looking out the window and at your calendar and knowing exactly where to go fishing today and what's running and what bait they will hit. Only experience will get it to be second nature.

EricV

I routinely use a little retarder.  I've used Sig retarder with several brands of dope with no problem.  Always a good idea to make a test on a non essential item.

I have used a plasticiser like castor or banana oil in the past and that seemed to slow down the evaporation of the thinners somewhat.

Or you could try "  SIGDR001 SIG DOPE RETARDER 4 OZ."

Trying to paint with dope here in Florida during the late spring, summer, or early fall without retarder without blushing is nearly impossible.  I've used Sig dope, with Sig thinner, along with about 10 % retarder.  Unfortunately my quart can of retarder leaked, which is one of the reason I switched to Rustoleum, and Duplicolor.