Modeling USN / USMC Weapon Thermal Ablative Coatings

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Since the Vietnam war US Navy (USN) and US Marine Corp (USMC) GP bombs are distinguished by a thick ablative fire-retardant coating, which is designed to delay any potential accidental explosion in the event of a shipboard fire. Land-based air forces typically do not use such coatings, largely because they add some 30 lb (14 kg) to the weight of the complete weapon. (Source: Wikipedia)

I'm always looking for ways to make my models that little bit more interesting, and it seemed that something as noticeable as the ablative coating shouldn't be that hard to reproduce. I never really tried to tackle it until I started my AV-8B Harrier. Around that time I was speaking to a friend at our local IPMS club and he was showing me how he applied a non-slip finish to his armour models. Figuring that the technique could be adopted for my purpose, I decided to give it a go. Thanks Mark :)

Step By Step
They say a picture is worth a thousand words, so lets get into it.

Figure 1 The first step is to mask off those parts of the weapon that should not have the ablative coating applied. Here I am using a GBU-16 from the Hasegawa Weapons set. In this case I had already painted the tail fins which is why they have been completely masked off using Gunze Mr Masking Sol neo.
Take your time in this step as having a sharp edge to the ablative coat helps ensure a realistic result.
Figure 2 The coating itself is made by mixing a liquid styrene cement (in my case Testors) with a modeling putty (here I have used Squadron Green Putty). I have not experimented with other combinations of putty and glue, but assume it should work the same. If you give it a go, let me know how it works out.
Figure 3 Now for the messy part. Squeeze a decent amount putty onto a clean surface (I am using a ceramic tile). Slowly add in small portions of the liquid glue and mix it into the putty with a toothpick. Once you get a consistency that is about that of honey, start applying it to the body of the bomb using the toothpick.
Figure 4 One thing I found was that this goop dries pretty fast, so you probably only have a couple of minutes to move the mix around once it hits the model. By the time I had finished applying an even coat to this 1/48 1000lb bomb which is about 1" (2.5cm) long, the first sections were starting to set. The morale here is, don't muck around. Get it on and use the time to smooth it out and get it looking the way you want. If the phone rings in the middle of this exercise, ignore it !!
Figure 5 At this point the goop has started to seriously set and grip the model. Having glue mixed with putty makes this stuff stick pretty well. Resist the temptation to try and fix any uneven bits at this stage as you'll only make a bigger mess. Once the coating dries you can even it out some by sanding it.
Figure 6 Give the putty/glue mix overnight to dry properly. Here I have applied a primer coat to see how it looks. Now is the time to sand any high points.
Figure 6 Once I was happy with the finish I applied an Olive Drab coat. Now you can see why I painted the tail fins first :)
Figure 7 Next step is to remove the tape from the tail fins. Be careful here. Don't just rip off the tape as it will bring off parts of the coating. This is because the coating is brittle when dry. To avoid this, you will need to use a sharp blade to trace the edge of the tape. This will separate the coating that is applied to the model and the coating that is on the masking tape.
Figure 8 All going well, you can now continue to paint, decal and weather your bomb as normal.
I think you'll agree this is a pretty easy method for simulating this distinctive feature of modern USN / USMC aircraft.

Good Modeling !

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