It Stinks, it’s Messy but it seals Oh -So Goooood!proseal A&P license

Proseal is a  common aircraft sealant initially used to seal aircraft fuel tanks. This sealant has proved to be so effective that it is now used on many other areas of the aircraft other than the fuel tanks such as the fuselage and anywhere else a sealant may be needed. It is important to know that there are many different types of pro-seal kits available and the term “proseal” is generic. Always follow the manufacturers recommendations when choosing an aircraft sealant. Also, follow the sealant’s instructions for mixing and follow all safety guidelines. Read the MSDS of any product before use.


What is Proseal?

Proseal is a two part sealant used in many areas of aircraft maintenance.  It is resilient to fuels which makes it a great fuel tank sealer.  Sheetmetal mechanics use this stuff when joining pieces of sheetmetal during fabrication or repairs. It is typically used to seal gearboxes and antennas from moisture.

This is not an exhaustive guide on aircraft sealants, rather a guide to get you familiar with the stuff and teach you a few methods of application to make your projects look professional.

Proseal  is sometimes called B 1/2 (B half) or B2.  B1/2 has a half hour work time and B2 has a two hour work time. If you are doing large areas then B2 might be better but I typically get B1/2 because I am not using all that much and I want the stuff to cure faster.

proseal aircraft sealant
CS3204 B2 Fuel Tank Sealer
Premixed Tube

Premixed tubes come in 3.5 oz sizes. They are convenient to use and easy to mix. Just push the plunger in a few hundred times and BAM! you got perfectly mixed proseal!  My friend has a little tool that he made which allows a drill motor to be attached to the plunger. This makes mixing REALLY easy. The downside is that if you only need a small amount of sealant, you will end up wasting lots since once you mix it, you have to use it. For small amounts this is not cost effective.

While proseal is a great aircraft sealant, it does have a  short shelf life.  Typically 6 months and you have to discard the old stuff.  Using old proseal runs the risk of not getting a full cure. Then you have a half cured proseal mess on your hands.

Mixing Proseal

Many aircraft sealants require  a 10:1 ratio mix. Always read the label to make sure. I tend to mix it a little “hot” because mixing it cold means it will not cure and there is nothing worse than trying to clean off old uncured proseal and reapplying.  Mixing it too hot can cause the sealant to become harder and you will loose some flexibility. If you want to be precise, you can use a scale to portion each part perfectly.  Most aircraft mechanics just eyeball it and go for a uniform gray color.  Again, follow the manufactures instructions. (disclaimer)

a and p license

"hmmm fresh batch of Pro-Seal...smells so good"

Mix it evenly with no yellow or black streaks.  Acid brushes are often used but not recommended because the bristles tend to fall off in the mix thereby contaminating your mix.  Instead I flatten the acid brush handle and use that end. Or use wooden stir sticks.

The manufacture recommends that you use PART A and B of the same lot number. This will ensure compatibility.

proseal A&P license mechanic
viscosity of peanut butter only more messy

When combining parts A and B, use separate sticks for each. This way you will not cross contaminate either parts.

proseal aircraft sealant
Mix it thoroughly

I always keep the used container for a few hours to make sure my sealant cured properly.  This way I can just check the unused portion of sealant for cured-ness.


Apply Proseal like a Pro

Proseal is messy to work with and can be somewhat tricky to make look good. Follow these simple tips for a clean professional look.


Use masking tape to create clean lines. Remove the tape after the mix starts to set up but do not wait until fully cured. It will be difficult to remove and you may have tape imbedded in the sealant.  This is always the point at which I wished I wore gloves.

smoothing it out

After the sealant begins to cure, you can then smooth it out and give it a more uniform look.  Wear latex gloves for this part. Wet your finger with windex or something similar. I have seen people use saliva with good results. (I know it’s gross but hey, it produces good results!) Smoothing out the sealant with your finger. It should not stick to your fingers because of the windex.


Perfection! (click to enlarge)

Continue to smooth out the sealant after removing the tape. Sit back and enjoy your professional looking sealant.


Homemade Applicator Method

If you are a much larger area that you need sealed you can create a professional looking bead of sealant using a large 60CC syringe. These syringes are useful for many things so keep a few stashed away somewhere.

Carefully fill the syringe with Proseal


I really don't love proseal all that much

You get lots of control when using the syringe method. This is a great way to create a professional looking bead especially if you have long runs to make. No fuss, no mess, when you are done, toss it in the trash.


BK117 tail rotor gearbox
Gearbox sealed using proseal using making tape method
Fuselage sealed using syringe method (non-pressurized acft)

Make it look Good

I look at proseal application much the same way as installing safety wire. If you do a sloppy job of installing,  people may think all your work is sloppy. You can put your heart and sole into an engine overhaul but if your sealant on the outside looks like a 3rd grader did it, people may question your abilities.  Any knuckle dragging mechanic can slap some sealant using an acid brush and get it everywhere, heck, it probably will even seal fine. But the job will look like crap. Take pride in your work, make it look as good as it performs.

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