Painting foam board

[AlSmith]

I'm new at this dollar Tree foam board building. No problem building them (5 and counting). I would like to know why everyone says to not use water based polycrylic Minwax. I sprayed 3 planes and did not have any problems. I used a Harbor Freight touch -up gun. No lifting at all. Just had to do minor sanding with 600 paper. This is normal with balsa planes.
Thanks, Al

 

[rcfred]

If you're just spraying a thin coat onto the foam board, there shouldn't be a problem with using water-based finishes. The problems that people have had come about when the surface of the foam board is flooded with finish. The water penetrates the paper, causing it to expand. This expansion can warp the model or cause the paper to detach from the foam.

To use a wood analogy, applying a finish to foam board with a brush is analogous to immersing a piece of wood with the same finish for an extended period of time. You can apply water-based Minwax to a sheet of wood without causing any warping. But if you were to soak the wood overnight in any water-based finish, the wood fibers would expand, and the wood would warp. You can soak a piece of wood in oil-based varnish and it won't warp, because the fibers in the wood don't expand when they absorb the solvents in the varnish.

 

[coldbloodedtx]

http://flitetest.com/articles/minwax-on-your-plane then add krylon!

 

[Stradawhovious]

^^^ This.

I minwax, then rattle can to my heart's delight.

 

[JasonEricAnderson]

The brush on technique is where you usually run into problems when using water based poly.

 

[Blenderite]

Personally I noticed that Minwaxing makes the foam brittle. Not sure if this was a mistake of mine or its a fact of minwaxing. But I don't plan on doing it again, unless someone can tell me what went wrong. I just brushed it on and wiped off the excess.

 

[nerdnic]

Personally I noticed that Minwaxing makes the foam brittle. Not sure if this was a mistake of mine or its a fact of minwaxing. But I don't plan on doing it again, unless someone can tell me what went wrong. I just brushed it on and wiped off the excess.

I had the same issue. I have since minwaxed a few planes and I think it has to do with how much minwax you use. The more recent applications have been very thin and I clean it off very fast and I have had better success than with my first few.

 

[hursto75]

I use minwax, really thin. Then after that drys I paint it with some thinned out and I paint it through a cheap airbrush. I got the airbrush set up from Harbor Freight for around $90. I use acrylic outdoor paint. The plane takes the paint great after the minwax. If you don't minwax you have to put on a lot of light coats. if you put too much paint to fast you will warp the foamboard. The best is to practice your airbrushing on a scrap piece of foamboard.

Thanks,
Brad

 

[SteevyT]

I discovered that using paper towels to brush on the Minwax rather than using a foam brush seems to get a much thinner coat applied.

 

[Blenderite]

I'll try getting a thinner coat on my next plane. That sounds like a very likely cause.