Fill, sand and repeat…
Continuing with refinishing our FMS P-51, in this installment we’re smoothing out the entire airframe including filling in all of the oversized panel lines. From there we’re applying our protective coats of polycrylic which will provide our surface in which we can do our paint preparation. If you missed my build review and paint stripping methods, you can catch that here. Let’s get to it!
It’s no secret, the panel lines on these foam warbirds are huge. When I took on this project, I knew that was one of the first things I wanted to rectify. The overall shape looks so good, smoothing out the finish would only make it look that much better. So, enter our light weight spackle! With the paint stripped off of the airframe, the process for filling in the foam texture and panel lines is quite simple. The key is in the material however.
When I first started this, I tried the pre-primered Patch ‘N Paint from Home Depot as it’s inexpensive to purchase and I just wanted to give it a shot. Unfortunately, the material did not work very well at all (I’m sure you noticed in the video!) in my opinion. It seems that the primer affects the consistency as the material does not dry hard (it has a consistency almost like a dried out marshmallow) and does not sand well (crumbles away more than sands). At one time, you could purchase the material without the primer in it and it worked well. If you can still find that, then it should work but unfortunately it appears that may not be the case anymore.
So, after disposing of that material in the best way I could find, I instead went back to my trusty Hobbico Hobby Lite balsa wood filler knowing that it would work well. It’s excellent material that sands beautifully. Applying a touch of water into it helps create a creamier consistency so that it smooths out over the surface nicely. In terms of the application, it’s quite simple as we’re just slathering it over the entire airframe. However, I like to use an Artist’s Spatula for the application. This is a very handy tool to have in your arsenal as it works great for applying all types of fillers and epoxies and being metal is easy to clean. So, using this, I simply applied the filler over the entire airframe…like icing a cake. Over the panel lines I tried to have sufficient body so that once sanded it would create a smooth continuous surface without any concavity to it. Of course, there’s always some touch up needed that usually pops up after sanding. Note that the prep time spent here will establish the smoothness of the finish once the polycrylic is applied. If you’re noticing popcorned foam and don’t fill it in, it will pop through the polycrylic and finished surface. So, an ounce of prevention is worth a pound of cure here.
SAND BABY SAND!
With the filler applied and thoroughly dried, it’s ready to sand. If this were a wood airframe, I would attack it using about a 12″ long sanding block to sand it all smooth. However, given that this is EPO foam, we want to avoid sanding into the foam as much as possible while also minimizing possible gouging. So, in this case, I used some 180 grit sand paper for the primary sanding task to knock it all down and alternated with a small 4″ sanding block to level out any peaks and ensure a continuous surface (the best way to sand a round corner is with a flat block ironically). Note that we don’t want anything heavier than 180 grit as we don’t want to risk cutting too heavily into the foam. Also, in this case I sanded primarily in a circular motion as that helped to localize the sanding and avoid sanding the foam. So, using these two tools, we can really quickly sand down the entire airframe and get it all smoothed out.
It will take a couple passes of the filler. Typically, as you sand you will find areas needing some touchup, so I recommend applying the touchup as you see it. Also, the time spent in this process here will help determine the overall smoothness of the airframe once the polycrylic is applied, so spend the time needed.
Radiator Vent Scale Detail Addition
One thing to note, the aircraft out of the box has 2 non-scale cooling holes on the underside of the radiator fairing. To remedy this non-scale feature, I instead cut into the airframe the scale radiator exhaust vent. An easy mod to do, I simply traced out the outline of the exhaust door and vent onto the underside of the airframe and cut them out. Using an X-acto knife while cutting vertically straight down, the exhaust door was cut out. The same was done for the exhaust portion of the fuselage too, however the top of the vent in the fuselage was cut horizontally (parallel to the waterline of the aircraft) so as to create an exhaust duct. Once removed, the top and sides of the vent exhaust opening in the fuselage were lined with 1/64″ plywood as shown in the pictures below.
The vent door was finished off by lining both sides of the door with 1/64″ plywood. The hole in the part was plugged with a piece of depron. Filler was then applied and then sanded to contour to finish it off. Once the assembly was completed, the vent door was glued into the airframe at the desired angle. A much needed improvement as this vent door is very characteristic of the P-51!
Elevator Stiffener & Pushrod Attachment
Noted last time was the fact that the elevator and rudder pushrod installations were lacking robustness — to the extent of having backlash (meaning that when load is applied to the surface, the pushrod bends rather than the surface deflecting). To resolve this, when the radiator exhaust vent was added in, I used that as a chance to fasten the pushrod sleeves down with some 5 minute epoxy and micro-balloons. There is a slot in the foam, so the pushrods were pushed into the slot when epoxied with a 1/8″ lite plywood cover to ensure they cannot break free.
The other thing I noticed is that when pushing large deflections, the elevator carry through lacks sufficient stiffness to keep the two elevator halves from twsting. So, to remedy that, I simply glued with 5-minute epoxy a piece of 1/16″ plywood cross-grain along the back of the elevator carry through. This stiffened up the assembly significantly and now no-longer twists at higher deflection angles.
ALONG CAME POLYCRYLIC
With our airframe completely filled and contoured, it’s time to seal it all up so we can get it ready for paint! The process is simple, I simply brush the entire airframe with multiple coats of Minwax Polycrylic (satin finish). Using a foam brush to apply it, the target should be about 5-8 coats or so to seal it all up and smooth it out. Try to avoid the door hinge areas as it does gum up the hinges. However, it’s not too terrible to remove once it’s dry if that occurs. An important note is to avoid drips as much as possible as they’re not easy to sand out. Unfortunately they can be difficult to spot since the poly is clear when it’s applied, so apply it mindfully. The nice thing about it is that it dries quickly (especially if placed in the sun for a couple minutes) and so you can apply multiple coats rather quickly. In our case here, I applied 6 coats of polycrylic onto the airframe.
With the polycrylic applied and cured, we now have a nice canvas for us to paint over while also leveling out our finish a bit. From here, we will begin to primer the airframe and prepare it for paint…and then of course paint it! Both of which we will cover next time. So, until then dust off those projects and get building!