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OK, so let's talk about thinner or reducers as we call them in the trade. As you may have noticed I seem to be against adding any reducers to coatings and as a general rule if you can get good spray and flow without that is what you should do. But there are times when a bit of something will make a big difference. There are retarders and there are reducers.

Reducers; I'm sure a lot of you just buy lacquer thinner at the hardware store or wherever you got your lacquer. And that can work, but you never know what exactly you are getting in that can.
Professional reducers are rated by evaporation rates. This correlates to temperatures. For most of my reducers I am using PPG. Here is a break down of them,
DT860-cool temp, 60-70 degrees. Fast evaporation rate.
DT870-medium temp, 65-80 degrees. This is a good wide spectrum reducer. It works very well in metallic base coats.
DT885- warm temp, 75-90 degrees and above. another good one to have on hand.
DT895- hot temp, 85 degrees and above. Better choice for high temps and two part urethane clears.
DT898- hot temps, 95 degrees and above. Slowest evaporation rate.

So why should you match your reducer to the conditions? You will often use less reducer which means less VOC's and less money spent on materials. You will also get better surface quality and optimize between coat times. Both of these are very desirable, less level sanding and less time spent spraying.

Retarders; these are different then reducers. A reducer works by making the material less viscous and lowering the solids content. Retarders actually slow the drying process. They are heavy solvents like Butly-acetate that blend in with the original solvent pack and slow the evaporation considerably. While they do reduce the coating a bit it is no where near the percentage that reducers do. That is because you use so much less. Typically on a given day if I would need to add 10% reducer I would only be addoing 3%-5% of a retarder. The retarder will not behave like a reducer. It will allow the coating to flow out but it will not help a lot with atomization problems. So knowing what you need for the air of the day is important. These things are only learned through experience.

You can get retarders from PPG, I most likely have some here somewhere. But I use products made by the company that makes my nitro. They call them Flow Enhancers and there is #1 and #2. I like the #1. It is mostly Butyl-acetete and Propanol-acetate. I can get good flow out on my nitro with just adding a small percentage of this stuff when temps and RH are high ( like a few weeks ago). As I said the coating behaves a bit differently then with a straight reducer. And it takes some getting used to. But just like the change from siphon guns to HVLP many years ago their will be those who complain and drag their feet but in the end it is a change for the better. The key is knowing what you need the coating to do. If you can use a retarder you should. If you need a reducer use the appropriate one.

Retarders have a lot of uses in finishing and we will talk about them and some other points as I go along here.

So now that we have the basics, let's look at some possible scenarios and which product to use.

Reducers;

Most sanding sealers for spray use are high build formulations. They are also fast dry as well to speed production. They work great out of the can at 70 degrees and 50% RH but as temp climbs problems will start. The first thing you will notice is you will not be able to maintain a wet edge and will start getting dry laps. As temps increase you will start to see white boogers in your finish, it's not dirt it's sealer that is drying before it gets to the surface. Eventually all you will get is sticky white powder. This is the time for a bit of reducerr. To keep the fast drying qualities you will want a reducer one step faster than the booth temp would suggest. Start off adding a small amount at first and only add enough to keep a wet edge. 15% should be considered a max. If you are still having problems check your equipment and if that is OK then add a bit of retarder with the Reducer. Another time I like some reducer in my sealer is the first coat if I am going to be using a paste type pore fill. This coat I will cut maybe 25% with one of my fastest reducers. The reason being that I use a high build sealer and I want only the minimum functional amount under my pore fill. This will help reduce shrink back of the pores in the finish.

Another time reducer is called for is when there is a lack of flow in the coating producing an unacceptable amount of surface texture or "orange peel" (Retarders can be used here also and we'll discuss that in a bit). It should be noted that some surface texture will always be present in an air dry atomized coating, it will never flow out like glass. Use the reducer rated for your booth temp and only add enough to restore flow out. 5-10% usually does the trick. This assumes that coating is being applied at the proper wet film thickness. Applying coatings too thin will cause lack of flow and is remedied by applying more coating. Gun set up and application technique will have a lot to do with surface texture as well. Low air pressure at the cap or excessive fluid flow will cause insufficient flow out. Low RH can cause lack of flow too. Reducers should only be added when everything else has been checked out and problems persist.

Reducers are also very handy in adjusting color intensity of transparent tints and toners. Metallic and pearl base coats that are usually not sanded because of the inherent problems benefit from being reduced as well. Reducers can also be used to create a bond coat in air dry coatings without adding material. Vinyl sealer for instance, if left to dry and then level sanded, can be bonded to the lacquer top coat by simply spraying a light coat of the appropriate reducer over it and waiting for it to flash and then apply the clear. This same technique can be used when applying a fresh top coat of lacquer to restore an older instrument .

Retarders;
Retarders and reducers are both solvents.The difference is in the speed at which they react and how "heavy" they are. Retarders can do some of the same things that reducers can in certain situations. There are some problems only they can readily solve and others where they are no help at all. To give an idea of what some of these chemicals are so you can identify them in different solvent blends, we would generally be looking for butyl's and glycol's. Butyl's are only suited for use in solvent based systems. Glycol's on the other hand can be used in water based systems as well, they behave much like alcohol only with slower evaporation rates.

Retarders are of little use in solving atomization problems.They require the use of reducers or more often the correct cure is to change needle and tip sizes, adjust air pressure, or otherwise address the issues at the gun itself. They also do little to improve a materials flow through the gun, some fan powered HVLP systems are sensitive to material viscosity by design.

They can be used to increase flow out of the applied coating. they work much different than a reducer in this application. Instead of the material being "wetter" and flowing out quickly and then flashing, the flow out is very gradual and will happen for a few minutes after the initial spray before it flashes. The advantages of this method are that it generally takes very little added to get the correct effect. Not only saving money in solvents and lowering VOC output. It means less fiddling with the gun because the viscosity of the material remains very close to the same. This means as my air conditions change over the day I can generally just add a bit of retarder here and there as needed and keep right on spraying without need to adjust anything. Consistency is one of the keys to good finish application. This can be helpful with those HVLP rigs I mentioned earlier. One disadvantage of using retarders in this manner is you will need increased dry times between coats. Applying coats to quickly will lead to solvent pops. We are not talking hours here, but under ideal conditions I wait 30-40 minutes between coats when doing this. But it should be noted I am using a high solids material which needs a bit more dry time to start with.

Retarder is the universal cure to "blush". Adding it to the lacquer before application will prevent it and spraying it straight over a "blushed" surface will remove it. You may be able to prevent "blush " with a slower reducer but retarder works better. Retarder can also be used to re-flow the coating and fill in solvent pops or other small pinholes and defects. It can be a good way to tie coatings together without sanding. It is also very helpful in some repair situations.

So that is most of what I know that is useful about these things. Please feel free to ask any questions and I'll do my best to answer them.

Wow, this is great information Brian! Thanks for posting this!!!

YES!
I did not mean to ignore commenting.
I have read and also appreciate; excellent information.
Thank you.

Kevin