The (somewhat) definitive guide to sealing foam and keeping it flexible

[forcetrainer]

EDIT: A picture is worth a thousand words, and without pictures this post is just really long and boring. The image issues were caused when the files were retrieved and stored locally by the forum. All the images have been changed to direct links to Imgur. This should fix the issues. I also

Long time reader, first time poster. It may seem weird that my first post is as long as this, but for me it's kind of a pay it forward to everyone here on TRPF. When I decided to start crafting foam pieces it was the posts here on TRPF that helped me get moving, so I wanted to provide the same type of insight to other noobs in the foam crafting world. Hopefully my labors will help guide others.

And yes, my title is absolutely tongue and cheek since this is in no way definitive There are a bunch of ways to skin a cat - this is just my journey to discover the best way for me to do it. Enjoy!!

OVERVIEW

My interest in crafting with foam started this year when I decided I wanted to try and put together a great Halloween costume. I decided on the Joker as part of a family theme, and specifically I decided to go for the look of the Joker from Injustice. One of the main differentiators (and my favorite part) with this version of the Joker is that he has armor on his shoulders. So obviously I needed to figure out a way to make that happen, and a few Google searches later I found myself researching foam crafting.

I quickly bought some EVA mats and mocked up my first set of shoulder armor. I was able to mold and shape the piece easy enough, but when I got to the point of painting the armor I hit a stumbling block. What's the best way to paint the foam? Do you need special paint or primers? How durable is it?

While I found many common themes among all the different methods of sealing and painting foam, most of the ideas were based on trial and error versus a side-by-side comparison of the methods. The OCD side of me wanted to know things like: why five coats of PVA instead of four? Is Plasti-Dip really as flexible as everyone says? Is there really a benefit to using Plasti-Dip on top of PVA? Is Plasti-Dip better by itself?

It's these questions that drove me to investigate the process of sealing and painting foam with the goal of finding the best possible method while at the same time reducing extraneous work and costs.

SCOPE

I need to start by saying this focuses on methods for coating foam that you want to remain flexible. XRobots did a great write up here in 2012 on how to plastic coat foam, so if you want fiberglass like pieces that's the way to go.

I decided to limit the scope of this experiment to finding the best/most economical way to seal the foam. There are a lot of variables when it comes to paint choices, but in the end a clean, smooth, flexible base layer is what is needed regardless of the paint you choose. While some paints will not tolerate any flexing at all, most paints will go along for the ride as long as what it's sitting on remains flexible.

There are a lot of competing theories on the best way to seal foam. The leading methods include either PVA, Plasti-Dip, or a combination of the two as a way to seal the foam. I know there are other options such as Mod Podge, but since Mod Podge is basically PVA with some additives I figured PVA would be good enough. I also know that most people add other additives to the Mod Podge to make it more flexible, and I didn't want to get bogged down in finding the perfect Mod Podge formulation.

METHODOLOGY

The overall purpose of the experiment was to determine the best way to seal foam and have it remain flexible. The idea of "best" is open to interpretation depending on what you're building, so I determined the following criteria would help to determine a general idea of "best":

• Ability for the sealant to smooth out sanding, cuts, gouges, and repaired areas;
• Smoothness of the final painted product;
• Flexibility of the final painted product.

Identical foam pieces were prepared with the following attributes:

• The pieces were divided in half with one side being sanded with 320 grit sandpaper and the other side not sanded at all;
• "Damage" was created on each side including: 2 x razor knife cuts, approximately 1" long (one starting on the edge and one in the middle), 3 x gouges similar to the pock marks found in the EVA foam, 3 x pin holes to simulate how some people hold down patterns with pins, and a 1"gouge that is repaired with flexible caulk.
• All pieces were painted with 2 x coats of spray primer and 2 x coats of blue, Krylon spray paint after the designated sealing method.

Four sealing methods were chosen:

• PVA (2:1 PVA to Water Mixture) - Iterations of 1 to 5 coats
• Plasti-Dip (PD) - Iterations of 1 to 5 coats.
• PVA, then Plasti-Dip - Iterations of 1 to 5 coats of 2:1 mixture, then 2 x coats of PD.
• PVA (3:1 PVA to Water Mixture) - Iterations of 2 to 4 coats. This method was added after the experiment was started and was reduced based on experience from the first three methods.

RESULTS

After approximately two weeks of preparing, sealing, sanding, and painting, I was finally finished. Once the blocks were finished I evaluated each one for 1) appearance and smoothness (both visually and to the touch) and 2) flexibility.

Appearance and Smoothness

PVA (2:1)

The PVA by itself is not able to fill in any of the marks or gouges, and the damage becomes more pronounced the more layers you put on. Even the pin pricks are still noticeable. I started sanding the surfaces with 1000 grit sandpaper from the second coat on, and the surface was relatively smooth. However, both visually and to the touch there was a noticeable pattern from the foam. Past 3 coats I couldn't see any significant improvements, so I would say that if you were only coating with PVA that anything above 3 coats is just a time sink.

PD

The results of the PD squares were about the same as the 2:1 PVA. The PD was unable to fill any of the damage by itself, so the gouges and cuts were still very noticeable. PD also has a very distinct texture, even when sprayed on the smoothest of surfaces. It can be sanded with high grit sandpaper, but the results are not that great since PD still maintains its texture. Just like with the PVA there was no significant difference with more than 3 coats.

PVA (2:1) and PD

I know that the PVA/PD combination is extremely popular, so I had high hopes for this combination. Unfortunately I was disappointed to get squares that looked identical to the PVA and PD squares. Since the PD sits on top, the squares all get the PD texture, and unfortunately the 2:1 PVA is unable to fill in the damage like I had hoped.

PVA (3:1)
My original scope was only for the first three iterations, but the disappointing results pushed me to look for another option. I knew that some people were using either straight PVA or PVA that was only slightly thinned, so I decided to go for a 3:1 PVA to water ratio. I also switched to using a foam brush instead of a bristle brush. That change proved to be much more significant than I thought. Finally, I didn't make a gouged area and fill it with caulk since by the time I got to these squares I didn't have any caulk lying around.

As you can see, the results of the thicker PVA are significantly different. The PVA was much smoother and it managed to fill in the damage much better. The larger gouges were still visible, but they were significantly lessened. To the touch the surface was completely smooth and it took 1000 grit sandpaper without any issues.

Flexibility

To test flexibility I decided to use the squares that had 3 coats of PVA. This test was fairly simple - bend the square in my hand and see what happens.

PVA (2:1)

I did NOT expect this to happen. When I bent the PVA I was greeted with a loud CRACK. As you can see the cracks were actually fairly deep, and the foam split in areas that were not cut by me.

PD

The PD didn't have a resounding crack, but you'll see that a lot of small cracks appeared across the square. The cuts that I made in the damage area were also widened when I bent the piece.

PVA (2:1) and PD

This square did much better than the lone PVA and PD panels, but the cuts in the damage area still split.

PVA (3:1)

Given the results of the other flexibility tests I wasn't very hopeful. Much to my surprise the 3:1 PVA did not crack at all. I bent the square much further than all the others and it held up. Thinking that maybe it was a fluke (glue not fully cured, etc.) I bent the other 3:1 PVA squares - no cracks!


RECOMMENDATIONS

I learned a lot about foam, PVA, and PD throughout this experiment, and there are definitely a few recommendations that I can make regarding sealing your foam.

• 3 coats is enough, anything more is a waste of time and money - Whichever method you choose, I never saw a difference on the fourth and fifth coats. I've read some people doing up to 8 coats with PVA and PD, and personally I think it's a waste. Whether you're using PVA or PD, 3 coats is enough.
• If you want perfectly smooth 3:1 PVA is the best choice - Some people may want a rough texture, so they can stick with thinner PVA formulations. However, if your goal is to achieve a perfectly flat surface you will need to fill any gouges with caulk or some other flexible material (remember, we're shooting for flexible pieces), sand the surface with a high grit sandpaper, and then use 3 coats of 3:1 PVA. This will get you the smoothest surface possible without plastic coating your pieces.
• If you need extremely flexibility, go with thicker PVA - The reason the 2:1 PVA and PD pieces cracked is because the first (and probably second) coats seeped into the foam instead of creating a sealant layer on top. This made the foam itself more rigid and it was the foam that ultimately cracked when normally it is very flexible. The 2:1 PVA was the thinnest of all materials used, and that caused the biggest cracks. If you want flexible pieces you need to maintain flexibility in both the foam AND the sealant. The 3:1 PVA strikes a happy medium of not seeping too far into the foam but not being too thick that it's difficult to work with.
• Sealant alone will not make foam perfectly smooth - PVA and PD are not capable of filling in imperfections unless they're extremely minor. If you have any significant marks, dings, damage, etc., you will need to fill in with a flexible putty/caulk before sealing (or between sealant coats).

I know that a lot of people use PD, but there are many that find it to be cost prohibitive or difficult to find. I bought my PD for about $6 at Home Depot, and I paid about $20 for a gallon jug of PVA at Staples. I have yet to use my entire can of PD, but I'm going to assume that my gallon jug of PVA will probably last longer than 4 cans of PD. Cost aside, I would say that the 3:1 PVA is your overall best choice for the following reasons:

• You can achieve smooth pieces. That will never happen with PD.
• It's far more flexible.
• PD in a spray can is noxious. You can get PD in non-spray can form, but I have not had any experience with it. As SMP points out below in his post you can brush on PD with good results. Perhaps I can do a 3:1 PVA vs PD brush on next.

For me, 3:1 PVA is the winner. Here's the best process I found for sealing with the 3:1 PVA:

1. Prepare your surface by filling any joints, damage, holes, etc. with a flexible putty/caulk.
   
2. Sand all repaired areas with 320 grit sandpaper.
   
3. Apply first coat of 3:1 PVA with a foam brush and let dry.
   
4. Sand lightly with high grit sandpaper (if you want glass like finish).
   
5. Apply second coat of 3:1 PVA before the first coat cures. You will actually feel the first coat pull a little under the brush which helps keep the second coat in place.
   
6. Sand again with high grit sandpaper if desired.
   
7. Apply third coat of 3:1 PVA before the second coat cures, let dry, and then prime and paint!

Depending on the size of your piece you can get through the sealing process in a single evening.

BONUS: TIPS AND TRICKS

Doing an experiment like this you are bound to pick up a few things. Here are some random tidbits that I learned:

• Use foam brushes to apply PVA - This is without a doubt the best single tip for working with PVA. You don't have to worry about brush strokes, they're much easier to clean up, and you can buy them by the dozen for a few dollars.
• Don't worry about bubbles in the PVA - Originally I worried that all the bubbles would ruin the finish, but as the glue starts to dry the bubbles will settle and smooth out. Just brush the PVA on as smoothly as you can and the bubbles will work themselves out.

• Don't be afraid to use thicker coats of PVA - A lot of people recommend using thin coats of PVA to eliminate brush strokes (#1 takes care of that) and to cut down on the PVA running down a piece or pooling. The thinner PVA actually runs more, and no matter how thin a coat of PVA you put on you'll find that it will run and pool on non-horizontal surfaces. For horizontal surfaces you don't really have to worry, so just brush it on and smooth it out. For horizontal pieces you will want to go a little thinner, but don't worry if you get pools or runs. They can easily be smoothed out with sanding.
• Sand between coats - If your goal is a perfectly smooth finish, this is a necessity. While drying PVA will pick up dust, dirt, and other particles that can make the surface rough. I use 1000 grit sandpaper to quickly smooth out the surface before my next coat. Also, remember that PVA is water-based! Don't wet sand it unless it's 100% cured (24+ hours of drying).

 

[Nerfenstein]

I just wanted to reply because no one else had and I can see the major effort you did with your post. I make a lot of things out of foam and at times use the PVA method, so it's great to see it outlined here for others. Very informative post and congrats on your first post

 

[Grimwood]

Yes! Thank you! This is a MUCH cheaper option than the Plastidip I have already purchased. Future projects will use PVA 3:1

 

[SMP Designs]

First off, cheers for putting this together! It's great to see someone addressing the need to keep the flexibility so your finish doesn't crack and get destroyed the first time you bump into something. That said, here's my "IMHO" 2-cents...

In my experience with PD, I always advise people to use the brush on and never the spray. It's much smoother and self corrects. My MK VI that I've been tinkering with is only sealed with PD (brushed on - 3 coats). That's it. Here's my blog on it (sorry, I haven't transferred it over here yet, so excuse the faux pas of the external link): http://smpdesigns.com/smp/eva-foam-experiment-pt-10/

You mentioned using flexible caulk - I used ElastoPatch and it worked beautifully. Adheres the the foam well, is not messy or noxious and the PD coats over it very nicely. The only caveat is, it's not sandable. However, I just pumped it out on a paper plate and used my finger to apply it while dipping my finger in a bowl of warm water to keep it pliable while I smoothed it out. In one of the posts on that blog I show some of that process.

 

[thejakemoore]

I don't know if it's just me, but most of your images aren't working.

 

[jordandau]

I'm having image issues as well. I thought it might be the work firewall. But excellent article, I appreciate the effort you put into this.

 

[forcetrainer]

Thanks for the positive comments! It was definitely a lot of work and at times tedious, but I figure putting the time in up front would make any future builds much easier.

SMP - Your Mk IV looks great!! One question I have is regarding the texture. One of my goals was to find a way to get a plastic like smooth texture without actually having to plastic coat the piece. The 3:1 PVA works pretty well, but imperfections underneath still show through. It looks like your MK IV has some texture to it. Is that a result of the spray can paint, did the PD have that texture, or is it just an artifact in the picture? Hopefully you don't mind, but I grabbed one of your pics to do a side-by-side with the 3:1 PVA.

All of the imperfections show up VERY noticeably on the 3:1 PVA in the picture, but to the naked eye in regular light it appeared to be almost smooth. Not saying that one is better than the other - just trying to get a sense of what results you're getting with the PD. Thanks for the link!

 

[SMP Designs]

SMP - Your Mk IV looks great!! One question I have is regarding the texture. One of my goals was to find a way to get a plastic like smooth texture without actually having to plastic coat the piece. The 3:1 PVA works pretty well, but imperfections underneath still show through. It looks like your MK IV has some texture to it. Is that a result of the spray can paint, did the PD have that texture, or is it just an artifact in the picture? Hopefully you don't mind, but I grabbed one of your pics to do a side-by-side with the 3:1 PVA.

Thanks! And, no prob on grabbing the pic. The helmet does have a slight texture to it. That was the first piece I made on the v2.0 of that suit and I hadn't figured out a little trick of using my heat gun to pre-seal the foam before coating. That really helps. I think the smoothest of the pieces were the arms, abs and codpiece. It also seems to differ a little with the foam. Even buying the same brand of floor mats from the same store, there's slight differences in the foam that can show through. Kind of like buying fabrics from different dye lots.

I'm going to try out your PVA method on my next foam project, but I'm still a brush-on PD advocate. The TOR armor in my avatar and pic in my gallery is made of foam, heat gun sealed and PD'd. I just used Mod Podge and craft paint to finish it.

 

[XRobots]

This is really well done, thanks for doing the research

 

[arctrooper]

Ha! This is absolutely brilliant! Thanks for posting!

 

[dkasai]

I did my first attempt at prepping my foam Iron Man suit last night and I followed the instructions here closely. Or at least I thought. My finished product was a nightmare. After fully drying next day it cracked everywhere like an egg. Luckily I only tested it on the the shoes and calf pieces before moving up. What did I do wrong? And more importantly is there any way to fix it? Help appreciated.

 

[forcetrainer]

I did my first attempt at prepping my foam Iron Man suit last night and I followed the instructions here closely. Or at least I thought. My finished product was a nightmare. After fully drying next day it cracked everywhere like an egg. Luckily I only tested it on the the shoes and calf pieces before moving up. What did I do wrong? And more importantly is there any way to fix it? Help appreciated.

Which method did you use - the 3:1 PVA? Did you get the cracking before or after painting?

3:1 PVA is definitely very flexible, but if you put paint on top that isn't flexible you can still get cracking (it's the paint cracking, not the glue). I just checked a piece that is a few years old (Joker costume with shoulder armor) and there are definitely some hairline cracks, but that's due to moving around in the jacket, bumping into things, etc. I poked a bit to see after two years it got brittle, but it was still relatively flexible. The piece was painted with Krylon, which may be more flexible than regular acrylic (not too sure).

I really need to do a test of the 3:1 PVA with various forms of paint to see what holds up the best, but I need to get more spare time!

 

[Mike J.]

The piece was painted with Krylon, which may be more flexible than regular acrylic (not too sure).

I really need to do a test of the 3:1 PVA with various forms of paint to see what holds up the best, but I need to get more spare time!

What kind of Krylon did you use? The "Fusion" branded kind?


-MJ

 

[forcetrainer]

What kind of Krylon did you use? The "Fusion" branded kind?


-MJ

I actually used the Hammered Finish. Oddly enough the piece came out perfect!

 

[dkasai]

Yes, I used 3:1 PVA. I actually haven't painted it yet due to the severe cracking with the PVA.
Did I use the wrong kind of glue? I don't know where you are located but nothing in the stores here sell anything named "PVA GLUE" so I bought wood glue from Home Depot which I heard is the same thing. And just to be clear 3:1 PVA I assume means 3 parts glue, 1 part water? I was also confused a little with the sanding in between. It says let dry then sand but apply the next coat before it cures. I waited until it was completely dry. Like 45 min. Any other ideas on what went wrong?

 

[forcetrainer]

In my tests I was using regular white PVA glue (i.e Elmer's school glue), so I think the wood glue might be the problem. They're both PVA glues, but the wood glue is usually more brittle. My guess is that's what led to the cracking.

For the formula, yes, 3:1 means 3 parts glue to 1 part water. What I meant about sanding was sanding after it was relatively dry, but before it was fully cured (about 24 hours). When you apply subsequent coats if it's not fully cured the top coats will bite a little more into the coat below and not run as badly.

See if you can get your hands on some of the white PVA (Staples sells it in big jugs) and test out the method on a scrap piece and see how it compares.

Good luck!

 

[dkasai]

That did the trick. White (Elmer's) Glue is flexible when it dries but the wood glue dries hard and cracks. Thank you for your help. I should have tested it on a scrap piece before going on to my actual armor pieces. Not sure how to fix those now. thanks.