Testing Foam Finishing Products



January, 2017

I’m working on my next robot and need a sturdy, but very lightweight material for the body.  I came across Smooth-On’s FOAM-iT Series of castable, rigid urethane foam and felt it would be a good fit for my needs.  I chose FOAM-iT 4 to get the balance of rigidity and weight I was seeking.

FOAM-iT is a great, light-weight material that can be used to form complex shapes for robot building, but it does have some drawbacks.  FOAM-iT is UV sensitive, and can be damaged by some chemicals.  In addition, FOAM-iT molds are not always perfect and may require a bit of repair work.  Foam finishing products solve these problems.  I tested 3 different foam foam finishing products and these are the results.


Foam Finishing Products Tested

  1.  Flex Seal – As seen on TV, this is the stuff you can use to paint a screen door and install it in the bottom of a boat without leaking.  It produces a rubbery finish that is very weather resistant and tough.  I selected the black, liquid (paint-on) option since the aerosol spray stuff would damage the foam.  It also comes in White and Clear.  Flex Seal Liquid comes in a 16 oz. can and can be purchased through select retailers like Walmart in the paint aisle.0107172141
  2. Hot Wire Foam Factory’s Foam Coat – There are several varieties of this.  I selected the “All Purpose” variety.  It comes as a powder in a 3 lbs. container which can cover 36 square feet.  Foam Coat is recommended for Styrofoam, but can be used to coat any foam.  There are some good tutorials on line for this product.  When I mixed 3 parts powder with 1 part water, I found this to have a somewhat gritty feel.0107172141a
  3. Rosco’s Foam Coat – This comes in 1 gallon or 3.5 gallon buckets and was a bit of a challenge to find a seller.  For testing, I really didn’t want to buy a full gallon, but that was the smallest size available.  It’s water-based, non-toxic and designed to coat Styrofoam or polystyrine, but can be used to finish almost any surface.  Foam Coat seems to be especially popular in theaters for set pieces.  0107172141b


Test Method

Using identical foam blocks I made during a previous experiment, I applied multiple coats of foam finishing product.  I prepared the blocks in similar ways to test each foam finishing product:

  • I used a knife to cut off one corner of each block
  • I used a knife to carve a v-shaped gouge into the side of each block
  • I drilled a hole into the top of each block using a 3/8″ bit
  • I used a knife to rough the surface on one side of each block

Prior to the test, I weighed each block.  After adding product to each block to achieve a roughly equivalent coverage, I weighed them again and compared the difference.

I read a tip online that after letting a product set for a few minutes, you can use a wet sponge to gently brush the finished surface to get a smooth finish without the need for sanding.  I tried with and without this approach to confirm it was effective.



I used the following criteria when judging the results:

  • Ability to rebuild the corner I cut off
  • Ability to fill the v-shaped gouge
  • Ability to cover the 3/8″ hole
  • Ability to smooth a rough foam surface
  • UV Protection
  • Paintable
  • Weather Proof
  • Weight Added – lighter is better

Other important factors:

  • Low Cost
  • Ready “Out of the Box”
  • Easy of application
  • Dry Time – less is better
  • Ease of clean up


Test Results

Results 1.JPG


Test Results Explained

  1.  Flex Seal – ultimately failed to achieve some of the required tasks.  This was messy stuff that dripped everywhere, but I was happy to see that paper towel cleanup was effective.  After 3 coats, it was unable to rebuild a corner or fill the hole.  Given about 10 more coats, I may have been able to accomplish both, but at the expense of a lot of added weight and wasted time (since Flex Seal has the longest dry time of the products tested).  While Flex Seal does offer the best weathering and UV protection of any product tested, it completely fails in other required areas.  Although it only added about 10% weight, that did not include a rebuilt corner or filled hole.  If I had continued to add coats to complete these tasks, the weight would have been much greater.  Based on how I understood this stuff to work, I’m surprised more didn’t absorb into the foam.  Despite the manufacturer claims, I could not get Flex Seal to accept Acrylic paint – it may be ok with other types of paint (see update below).  Finally, this was the most expensive product tested.  Definitely a disappointment.0108171212
  2. Hot Wire Foam Coat – This product performed well overall.  It was the least expensive of the 3 products, and met most of the primary and secondary requirements.  The main drawback to this product is the need to measure and add water.  It offers a short set time (10-20 min), but takes quite a wile to fully cure (12-24 hours).  Without the wet sponge, this left a bumpy surface that took some effort to smooth with 150 grit sandpaper.  With a wet sponge, the surface was relatively smooth, but still a bit gritty – maybe that’s preferred for paint adhesion?  The biggest benefit to this product was that I felt I could control it best by adjusting the amount of water used.  I could get it thick to fill cracks, holes and rebuild the corner, but I could also dilute it to get a smooth finish and give myself a bit of extra work time before it set.  I’m not sure about the shelf life of the other 2 products tested, but this one should be excellent since it is in powder form.0108171212a
  3. Roscoe Foam Coat – This product performed well overall.  It was the lightest weight of the 3 products, and met most of the primary and secondary requirements.  The consistency was smooth and I found it easy to work with.  Without a wet sponge, this left a bumpy surface, but it was fairly easy to sand it down with 150 grit sandpaper.  With a wet sponge, the service was smooth and ready for paint.0108171212b



It’s a close race between the Hot Wire Foam Coat and the Rosco Foam Coat, but I’m going to name the Rosco brand the winner primarily because of it’s ease of application (pre-mixed and ready to go), longer set time, and quicker drying time.  It also produced good results while adding the least amount of weight to the foam block which is important for my robot project.


Lessons Learned

  • The wet sponge approach for smoothing the product worked great.  The trick is to find the right time to do it – I’m sure it varies by temperature and humidity.  A few times, I attempted to smooth too soon and ended up creating new line patterns that had to be smoothed later.
  • Clean brushes immediately after use.  Delay will result in loss of the brush.  I was using a foam brush and they turned hard as a rock after a few hours of drying.  Attempting to scrape off the dry product resulted in tearing of the foam.
  • Outside temperature is a consideration when shipping pre-mixed product like the Roscoe Foam Coat.  I was warned that the product could freeze in transit which could spoil the batch.  I rolled the dice, and luckily, it was fine.  Obviously, this is not an issue with un-mixed products like Hot Wire Foam Coat.



I wrote the manufacturer of Flex Seal to ask what kind of paint to use, and got this reply:

Dear Valued Customer, 
 Thank you for contacting us. Please be informed that Flex Seal Liquid cannot be 
painted over. If you attempt, it would need to be a high quality oil based paint. 
If you have any questions please reply directly to this email. Thank you and have 
a wonderful day.

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