Polyurethane treatment of dollar store foam core

The Experiment

Here are the results of a series of tests I did applying the Minwax (OIL BASED) satin finish polyurethane  to the foam core before painting. In the following video I provide a quick demonstration of how I applied the polyurethane to half of a scrap wing while leaving the other half untreated. After the treatment I allowed it to cure for one hour outside.  I then masked and applied the following:

1. Premium, brand name spray paint
2. Cheap $.99 spray paint
3. Brush on artist's acrylic (as an example of what not to do)
4. Misted an unpainted portion with water and left it on for about five minutes before wiping it dry with a cloth

Base Application

After an hour of curing you can clearly see the shift in the white color of the foam core. 

Here we are examining the treated side under a microscope. The paper fibers are still clearly visible but there is a noted smoothness to the surface.

When I masked off the piece for the polyurethane application I used 'normal' tan paper masking tape at full adhesion. As a result some of the paper fibers were pulled up when the tape was removed. 

Here you can see the treated side on the left and the untreated side on the right under the microscope. 

Premium and Cheap Spray Paint

I then masked off and painted a top stripe in green with premium spray paint and a black stripe with cheap $.99 spray paint. Both were applied with a fairly light handed mist but with no extended drying between the light passes of spray. I made sure there was no pooling or runs and painted it with the material vertical. This is my 'generic' spray paint technique and I wanted to see how the foam core would handle it. 

Both the cheap and expensive spray paints came out looking really good on both the treated and untreated sides. There is a difference in the sheen of the colors. The untreated paper retains a dull or matte finish with no real glossiness. The paint on the treated side did have a bit more sheen but I would call it the satin finish and no where near a gloss. 

What Not To Do

As a demonstration of what not to do, I painted a stripe of red artist's acrylic with a 1" brush. I applied the paint directly from the tube with no extra water added. Even without extra water the paint soaked into the paper so much that when I pulled up the painter's tape it tore much of the untreated paper with it. 

It is worth noting however that the treated side had none of the adverse reactions to the paint. The treated side maintained a crisp masked edge and the tape didn't bother it at all. A few hours after the red paint had dried I even scratched my thumbnail across the whole treated red stripe to see if I could get the acrylic paint to chip or flake but it held fast without a scratch. 

Water Damage

The tape for the brushed on acrylic paint had pulled up so much of the untreated side that I had to mark off a clear bottom section for the final water tests. For this I masked of the painted areas and used a spray bottle of water to mist the entire bottom portion of the wing with a heavy dose. I squeezed several pumps worth of mist hitting both the treated and untreated sides with the same sweeps. 

I then set the piece down for about five minutes before I took a dry rag and wiped the entire misted surface dry. 

The treated side beaded the water and it just sat there until I wiped it off with no damage to the surface itself. The untreated side wicked the water up like a sponge and by the time I had ragged the water away the paper had started to pull up from the foam. 

Observations

• Applying the Minwax OIL BASED (in all caps to make sure people don't buy the water based) polyurethane using the technique I demonstrated in the video sealed the surface without warping or distorting the structure in any way that I could observe. 
• Both the premium and cheap spray paints worked well with what I would call a 'normally careful' spray paint technique
• Painting the untreated foam core with heavy handed brush on paint (like I did here) is pretty much an automatic disaster
• Using a middle of the road (not cheapest not most expensive) artist's acrylic paint directly from the tube on TREATED foam core can work out ok
• The treated side repelled the water completely, it just beaded on the treated surface without soaking in at all
• Water will kill untreated foam core, period. 

Recommendations

I would recommend the Minwax treatment to anyone who is building models with this type of dollar store foam core, even if they don't intend to put a fancy paint job on it. A treated model would shrug off any mist or dew that I might encounter without any damage to the paper. 

Personally I'm going to be treating all of my scratch builds with this technique.

Recommended application technique in a well ventilated area with proper safety observations:

1. Get all your parts punched out from your speed build kit (or cut out)
2. Use a 1 or 2 inch synthetic brush to lightly but completely apply the polyurethane to one side of a large part or a few small parts
3. Avoid pooling and runs, approach it as 'wetting' the paper
4. Use a rag or paper towel to brush over the entire side that was treated to pull off any extra. You want to avoid leaving puddles or large shiny areas
5. Allow the treated parts to fully cure for at least an hour
6. Flip the parts and repeat the technique on the other side
7. For clean up use mineral spirits to clean your bursh, again in a well ventilated area and dispose of any soaked paper towels or rags safely

Oh and at the end of the video I thanked Flite Test and I was thinking 'Chad' but for some reason said 'Chris'. Sorry Chad :-)