Difference in Class A & Class B Sealants

I was researching MSDS sheets yesterday and found that the only difference listed in Class A and Class B sealants is that the Part A of the Class A has 10% Toluene by weight added. If that's the case, then what some have said of adding <15% Toluene by weight to thin Proseal should be perfectly acceptable, right (or at least <10% anyway)? Reason I ask is that I've contacted Flamemaster and PPG about doing this, and neither said thinning their sealant is acceptable, even though that is apparently what they do to make Class A sealant. I'm guessing that when they do it they are making a known quantity that they have tested, so they can validate the results through testing. When the homebuilder does it, the manufacturer has no control of what someone might do and how it might affect the sealant.

Am I correct? I want to make a final thinned batch of Proseal to paint over the seams and rivets so that I can look at it and say "it won't leak" before closing it up. Proseal is cheap!

You are pretty well correct in your assumption..the OEM will not likely give you data they can't "control". I work with Proseal almost every day..sealing up a 767 rear spar repair as we speak..I use MEK to thin B sometimes, and it works fine.its good practice to save a sample and verify it has fully cured. In the industry, we do timestamped cards that are part of the inspection. Its hard to mess up Proseal unless its old, or you don't mix it thouroghly. The thinning process evaporates out quickly.

Only use Class A sealant if you plan to fly above FL 180 while IFR. If you don't plan to fly this high, it is a waste of money and added weight.

Use class B if you plan to fly into busier airports, VFR or IFR, but keep in mind, you will need a specific clearance to enter!

What? You didn't mention class C. You are correct that the viscosity of each class is the main difference. The screwy thing is, the viscosity ratings are not intuitive. Class A is "brush on" [thin] sealant. It's used to cover rivet shop heads in fuel tanks and similar. Class C [medium] is what is called for to seal a fuel tank or pressure vessel [pressurized fuselage]. Class B [thick] is the most viscous, and is intended to fill large gaps.
Vans supplies us with class B, because you can thin B to make either C or A. FYI lacquer thinner is 90+% toluene, so it works very well as a thinner. MEK works, but evaporates more quickly. I prefer to use Toluene or lacquer thinner, as Toluene is what the manufacturers use as a thinning agent.
Charlie
PS You can triple the shelf life of ProSeal if you store it in a refrigeration.

Hmmmmm, wish I had read this before now. Just bought a gallon of Toluene on the way home -- smallest quantity I could buy, but I have a quart of Lacquer Thinner on the shelf. A gallon of Toluene will cover me for about 50 lifetimes of building. I tried to see if Xylene was equivalent to Toluene, but never could make that connection. Lacquer Thinner equivalency is good to know.

So, how do you make Class B into Class C, i.e. what % by weight of Toluene would you add, and is there any point in doing it? I figure I'll paint the heck out of the rivets and joints with my homemade Class A and call it good.

Class C sealants also typically are used when extended application/assembly times are required (C-8, C-24, C-36 etc) these extended set times are not available in the A or B class sealants.

Make sure you do a very thorough preparation before applying a top coat or it will just peel off (does peelng slosh ring a bell?).

Walt said:

Class C sealants also typically are used when extended application/assembly times are required (C-8, C-24, C-36 etc) these extended set times are not available in the A or B class sealants.

Make sure you do a very thorough preparation before applying a top coat or it will just peel off (does peelng slosh ring a bell?).

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Walt, if I apply it while the original sealant is still tacky, is that sufficient? What other prep would be required? Sure don't want any peeling going on.

Pro Seal & Toluene

Is anybody else concerned that toluene has been used as an octane booster in 100LL since it replaced straight 100 octane? Is Pro Seal supposed to last forever? Mooney owners think their Pro Seal sealant is good for 5 years.
Perhaps builders who fly their airplanes frequently have less trouble with leaks because the fuel stays fresher? I was shocked to discover that toluene was used in 100LL after using it to thin Flamemaster applied to my fuel tanks.

Sincerely,

Craig Evans

Stalldog said:

Walt, if I apply it while the original sealant is still tacky, is that sufficient? What other prep would be required? Sure don't want any peeling going on.

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I would call PPG for specific application instruction for the sealant you are using.

Walt said:

I would call PPG for specific application instruction for the sealant you are using.

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Walt, thanks for the link. I contacted them and was routed direct to PPG Aerospace Engineering Tech Support for fuel tank sealants. Here is what they said about applying the Class A over Class B.

1. If the Class B is freshly applied and not fully cured, i.e. not necessarily tacky but still pliable, and the Class B is still clean and has not been contaminated, then applying Class A directly over it does not require any further prep. It won't hurt to run a rag with MEK (PPG's recommendation) over it (Acetone will work, but it evaporates faster), but this step is not necessary if you are confident the Class B is still clean. Also wipe down the adjacent metal surfaces with the same solvent as a precautionary measure.

2. If the Class B has cured (approx 7 days), then lightly wipe the surface of the Class B with Scotchbrite, then wipe down with solvent (MEK or Acetone), then apply the Class A over the Class B.

3. In either condition, after the Class A has fully cured (approx 7 days), they recommend verifying good adhesion to be sure both coatings fully adhered. They don't expect anything other than full adhesion if applied as listed above, but this check will provide an additional level of validation. They didn't say exactly how to test this, but some sort of pull test. I will probably just apply the Class A over the Class B batch samples I have curing, then try to delaminate them after the Class A cures.

These steps only apply to virgin applications that have never seen fuel or other contaminants.

Using Xylene to Thin Proseal

Thought I would post one more response, this one from Flamemaster. I questioned them on thinning Class B, and also about maybe using Xylene instead of Toluene since Xylene is easier to find. Here's their reply:

"You are correct that MEK is not acceptable for thinning. While Xylene will thin the material the resulting solvent blend because of the differences in evaporation rates is subject to the formation of micro bubbles which may appear in the cured material.

Also keep in mind that class B material is a thixotropic formulation and no amount of thinning will make it properly brushable and/ or self-leveling.

For over coating B with A (brushable) the best recommendation is to the class a material which is truly brushable and self-leveling.

Respectfully,

Ken
--
Kenneth F. Chenard
Vice President, Sales
Flamemaster Corp
Nadcap AS9100 ISO9000"

So, my take on this regarding using Xylene to thin Proseal, unless one wants porous Proseal, don't use it.