Hardcoating Foam

Bondo on foam hardcoats

Hey guys,

I am a pretty beginner builder and I had no clue what to apply to my foam project to get a good hardcoat, so I began experimenting. The 2 big winners for me were “Durham’s water putty” and Bondo. In this project, I was applying on “great stuff expanding foam” and that pink foam board from home depot. I was really looking for something that would be nice and smooth, not too heavy, and wouldn’t flake/chip. Also, this was a pretty big sized project. Here are some of my results:

1. Speed Patch -

No…Just no. This is for repairing driveways so I thought it would be a good tough/weather resistant coating (plus I had a can in the garage). This stuff takes a long time to dry. It is flaky, heavy, and it is sandy. This will not give you a good smooth finish, and it will chip off.

2. Liquid Nails -

No, I love this stuff but its way too soft/flexible. In the past, I have used this for cardboard projects and had good results, but it is not rigid enough. Even if it was a little harder, it would gum up the sander if you wanted to get a smooth finish.

3. Flexseal/Truck bed Liner -

No. Both of these melted the foam.

4. Plaster -

Easy to use, very heavy, takes a long time to dry, brittle. For this, I used the premixed plaster (not Plaster of Paris). Compared to Bondo/Durham’s, this stuff is 2-3x as heavy.

5. Monstermud (mix of plaster and paint) -

I was actually pretty happy with the results of this one. It worked very well with the pink foam board. It sands pretty easy and gets smooth, but, again, it is really heavy.

6. Bondo -

This was one of my favorites. Easy to use (I had never used Bondo prior to this project and it was really easy to use. Once you mix it, it is almost like a fast drying clay). It is also very light, very strong, and cures fast. But, it requires ventilator (very strong odor), and is expensive. I used Body Bondo filler 3.

7. BondoGlass -

I only had a small can of this, and it is very expensive...That being said, if you can afford it - use it. Super easy to apply and get a very smooth finish. Very, Very strong and not too heavy. Basically just like Bondo, but it is stronger and easier to get a smooth shape (so less sanding afterwards).

8. Fiberglass -

This was the first time I ever used it..and I used the full can. I was a little intimidated, but I worked through it. You definitely need gloves and a respirator for this. Fiberglass is super itchy and I can only imagine what happens if it gets in your eyes/lungs. I used the fiberglass mat, but I think it might have been easier had I used the fiberglass cloth. This was a little challenging to use, but it is the strongest of all the hardcoats I tried, and you can get a nice smooth finish from it. This is a bit more time consuming then Bondo.

9. Durhams -

Easiest to use, smells good, very hard, light weight, cures fast. I read somewhere (maybe on here) to mix in a little wood glue and that’s what I did and I got great results. I plan on using this in the future. I really like this stuff. It’s also good for casting/molding, you can really get a lot of details into it.

Hope this helps. Like I said, I am new to this and still learning.

If you are interested, I made a video. Youtube search for: DIY chainchomp Catbed (nothingtodocrew).

So... what do you think? Please leave me a comment.

One Comment:

  • Vale: Great post! Thanks for taking the time and sharing your experiences with the different products, it’s really helpful!

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