Polyurethane treatment of dollar store foam core

by JasonEricAnderson | June 23, 2013 | (43 Ratings) Posted in How To

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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
The results and my observations are below

 

 

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 :-) 

Comments

Strix on June 25, 2013
Great article! I especially liked the microscope views. Nice work.
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RoyBro on June 24, 2013
Oh and great review BTW.

Thanks for sharing.
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RoyBro on June 24, 2013
It sure looks a lot easier than covering with carton tape ala Experimental Airlines. I just got a box of carton tape from Tape Brothers, so I'll be trying both.
I'd also like to try covering uncut foam board with the Minwax and then using a light spray adhesive on the back of printed plans to adhere them to the foam board. I'm hoping that the Minwax will allow the plans to release from the foam board after cutting.
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jhitesma on June 24, 2013
I've been using spray adhesive to attach plans for cutting and have had no problems with it on untreated board. I just treated 2 boards last night and will try the spray adhesive tonight cutting out parts to see how it goes - but I don't anticipate any change. If anything I'm thinking it should release a little easier. (I'm using the Krylon adhesive spray currently. Previously I used one designed for stencil painting that came in a much smaller can but seems to have been an almost identical product.)
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eagle4 on June 24, 2013
when treating your foam, what about treating the edges of the foam board? Also was there much of a weight increase in the treated v untreated foam? Fantastic article by the way. thanks for that mate
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JasonEricAnderson on June 24, 2013
The edges of the paper were treated when I brushed on the polyurethane but I didn't focus on the styrene foam at the edges. For my builds I'm going to be applying the treatment after I get all the cutting done. Specifically after any hinges, bevels, and any 50% cuts, that way I know any open paper fibers at the edges will be treated.

I din't notice any real added weight but I plan to get a digital scale and I may do a followup article for treating and painting my latest nutball airframe and I'll try and weigh it before and after the polyurethane treatment as well as after final painting.
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Orcoz on June 24, 2013
You mentioned several times that you strongly recommend against using a brush 'ever' for a model plane... while I agree that the ideal painting method is to airbrush, or to use light coats from a rattle can if necessary, I have used brushes for many of my planes with great success.

If using a brush, light coats are extremely important, as well as to let each coat dry completely before adding a second. The weight can add up quickly - but it isn't quite as taboo as you're making it sound.
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JasonEricAnderson on June 24, 2013
This was more of a "Don't do it like I did here" statement. I do note that even with the heavy handed brushing I did as a torture test it still worked well on the treated side. I was testing this treatment to see how it held up to "Bad" brush technique.
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leadpipe58 on June 24, 2013
Great work.I have general finishes urethane.I will give it a try and report back.It should be fine.
Keep up the good work..
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joann69 on June 24, 2013
Great article. Thanks for sharing. I will be getting some polyurethane "oil based" real soon to cover my foamboard planes.
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jerimiah85 on June 25, 2013
5 STARS and im stealing your pain job for my ft flyer.. thanks for sharing
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Miracle Air on June 25, 2013
I've been using packing tape, but after this I have to give paint a chance. Very useful article!
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amtpdb1 on June 25, 2013
Is there a problem with Packing tape or the Hot glue sticking to the treated board?
Thanks
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sailorJohn on June 25, 2013
Great job. I wonder if I could treat a ready built model (FT3D) with painted stripes (h2o shortcuts krylon) an not cause a problem such as running or smearing , best to test it first.
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EDLITE on June 25, 2013
Has anybody used Dollar Tree gift wrap? it comes in highly reflective colors and very visible from a distance! I am sure somebody else uses this as well but for those that have not used it.
I remove paper covering on foam board before I even start. I don't care if you use tape or paint; you are still relying on foam board paper and you see how easy it comes off. I lightly spray on adhesive and wait for glue to dry. I then apply gift wrap. WARNING: If you spay too much or to close with adhesive it will attack foam. If you apply gift wrap without letting glue dry it will attack foam. Done right it works great for me w/no additional weight. Practice first on scrap!
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Okie on June 27, 2013
when treating your foam, do you also treat the back of the foam boad
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JasonEricAnderson on June 27, 2013
Correct. I'd usually treat one side at a time. I'll have a demo up soon where I treat and paint an assembled nutball airframe.
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captainjerry on December 24, 2013
What about spraying your poly on in very light coats after you have assembled the airframe? So you don't have to wipe off the excess. It would also make construction easier since you won't have to remove poly coated paper to glue in things like . . . servos or hard points for landing gear, you know, that unimportant stuff:D




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skelator on June 27, 2013
i think water base still the stuff for depron as it does not have paper backing like dollar tree foam board
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Dirt Pilot on June 26, 2013
Good review. Out of curiosity, what happens when you use the water based Minwax? Did you weigh a piece of board before and after the application? How much weight per square foot does the Minwax add? Thanks.
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Willsonman on June 27, 2013
WBPU will peel up the paper while it dries and warp your foam. I do use WBPU for my coverings but I use tissue paper. After the initial covering I sand the bubbles away and do another couple coats of WBPU and give it a final sand. It makes for a very hard and durable surface that will accept any kind of paint. I use BEHR household latex paint from the Home Depot. I can get it color matched for any of my projects and thins well for an airbrush.The Behr paint is a Paint+Primer so it adheres very well and goes on with one coat keeping it light.
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Bob Post on June 27, 2013
Is anybody spraying the poly? I have sprayed poly for years on furniture and love to work with it. I also want to try shellac on the FB, anybody done that? For those who try to spray it, don't mix the poly and water based paints in the gun, it doesn't mix well and cleaning is never perfect. Use two guns.
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Captain Crash on July 3, 2013
where can you get the minwax

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JasonEricAnderson on July 5, 2013
In the US you can get it at just about any home improvement store, Lowes, Home Depot, even Walmart.
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Tightknot on August 12, 2013
I have covered dollar tree foam board with dollar tree plastic gift wraping. The wraping material is available in translucent colors and in metalic colors including chrome. I applied spray adhesive to the gift wrap and also to the board, in light coats. It is helpfull to have some help when laying down the wrap to avoid wrinkles but you can work wrinkles out with a credit card. The surface is very tough and good looking if you apply the wrap with the colored side down. One side of the clear plastic has the color treatment on it. To much adhesive can play tricks with the color finish, but if you spray the clear plastic side and put the colored side up, then the color scratches off easily. Put the colored side down to the board. I have applied the wrap to wings before doubling over the board at the wing leading edge. You can paint or do about anything to the board after it is covered with the plastic wrap.
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wga22 on October 22, 2013
Great article.
I have been using Thompsons water seal with good results as well. It just happened to be what I had on hand. It dried very light and gives protection against dew.
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Willsonman on January 15, 2014
Ok, so I have a question. How would this work if you were to remove the paper backing and apply tissue using the OIL-BASED poly? Would it cause major wrinkles? I am thinking out loud here. I want to print a lozenge WWI camo pattern onto tissue then apply it to the foam. Using the WBPU causes wrinkles that would mess up the patterns.
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coldbloodedtx on April 3, 2014
I just did this on my swappable super cub (turned L4), it worked out great even though I applied the Minwax after it was built. I was able to get the paint laid on nice and thick without any distortion. Some of the paper on the edges/corners wanted to separate a bit. Run a covering iron over those edges for a few seconds and it bonds it really well.
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tinkerbill on September 28, 2014
What an excellent wealth of info from both the author as well as other posters!
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JackRickman on December 9, 2014
Has anyone tried the wipe on the Minwax wipe on polyurethane?
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