Paint after Minwax still warping

[Foamforce]

I tried Minwax (oil based) followed by airbrushed acrylic for the first time in a Nutball. I brushed the poly on thick, let it soak for a few minutes, then wiped off the excess with paper towel. That was a cleaner and faster process than I expected, so that was good. I let that dry for 24 hours.

The next day I put on a coat of yellow acrylic. I put it on fairly heavy and my paint was maybe a bit too thin, but not so thin that I had any dripping. There was zero delamination of the foam board, so that was good. I painted a non-poly’ed piece of scrap as a control subject, and that badly delaminated, so I can see that the poly was very helpful. However, I was disappointed to see that after the paint, the Nutball warped a little. Just enough that the elevator became notchy.

What do you think I could do differently to prevent the warping? Do others use multiple coats of poly? Maybe I just made it too wet with the thin paint? Also, I realize the nutball is the worst case scenario because it’s a single flat sheet, but if I can avoid warping this, then I can avoid warping any plane.

Thanks!

 

[Foamforce]

Also, how well does the FT foam boards resist warping? Will it behave similarly to DTFB with Minwax, or does it do even better? I might pick up a couple boxes at FF23 to avoid the shipping expense. 🙂

 

[Inq]

I took the advice of the forum to use the Minwax oil based. I used a foam brush and didn't wipe it off. It settled out pretty well and it didn't warp. However, it was on a FT Storch which has multiple layers and/or 3D structure to keep it from warping, but the Stab/Elev and Fin/Rudder are single layer DTFB and didn't warp.

Edit: However, I haven't painted it yet. I plan to use just Krylon spray paint. Did you fully seal the edges so none of the paint could seep into the edges and cause your warp problem?

 

[danskis]

Its a fact of life: $tree foamboard warps. I use single layer elevators on all my planes to keep the weight down in the tail. To keep them from warping I glue a bbq skewer to the underside of the part. The skewers aren't straight but they always have a flat side you can put on the part. If the local hobby shop had more carbon fiber I'd use that instead.

 

[Foamforce]

I took the advice of the forum to use the Minwax oil based. I used a foam brush and didn't wipe it off. It settled out pretty well and it didn't warp. However, it was on a FT Storch which has multiple layers and/or 3D structure to keep it from warping, but the Stab/Elev and Fin/Rudder are single layer DTFB and didn't warp.

Edit: However, I haven't painted it yet. I plan to use just Krylon spray paint. Did you fully seal the edges so none of the paint could seep into the edges and cause your warp problem?

Yeah, I sealed all the edges with hot glue and a decent amount of the Minwax got in there too. It didn’t warp until after I painted it. Next time I think I’ll try two coats of Minwax, or maybe just not wipe it off as you suggested.

 

[Foamforce]

Its a fact of life: $tree foamboard warps. I use single layer elevators on all my planes to keep the weight down in the tail. To keep them from warping I glue a bbq skewer to the underside of the part. The skewers aren't straight but they always have a flat side you can put on the part. If the local hobby shop had more carbon fiber I'd use that instead.

Ah, that makes sense. Yeah, on the stabilizers I get warpage too. I’ll try the skewers next time.

Fwiw, the warp wasn’t bad, and sitting it on a couple blocks with a weight to counter-bend it got it back to “good enough”.

 

[Inq]

Yeah, I sealed all the edges with hot glue and a decent amount of the Minwax got in there too. It didn’t warp until after I painted it. Next time I think I’ll try two coats of Minwax, or maybe just not wipe it off as you suggested.

I'm certainly not implying I know anything. I'm in my first month or two using this foam board design process. Way cool! I only know Poly is damn near bullet proof (and waterproof) from years of making furniture and even boat cabinetry that children/people drench with water regularly. Never, ever anything gets through if seal completely (especially oil based). I'd like to try painting with air brush and acrylic when I get good enough (flying) to start making war birds, so I'll heed your troubles and watch this thread for other (more expert) advice.

 

[luvmy40]

Of the few models I have actually painted, none of them have had any noticeable warping from the paint.

A couple have been FTFB and a couple have been DTFB w/minwax. I brush the oil based minwax on with a natural bristol brush, lightly and wipe off anything that looks thicker than the rest,. Let the minwax dry over nigh before painting. Paint with multiple light coats, letting them dry between coats. I have not used any acrylics but I have done both rattle can enamel and airbrushed latex.

 

[Foamforce]

I’m interested if something like this could be used to wrap the exposed edges and provide some rigidity. The trick would be finding the exact right size. I think the .188 is the closest. After removing .032 for the flanges it’s down to .156, which is about 1/32” smaller than foam board. It might fit with a pinch.

https://evergreenscalemodels.com/products/266-188-4-8mm-opaque-white-polystyrene-channel

 

[bwarz]

Wood glue on the edges seems to hold up better against the minwax for me. Wiping it off is always a good idea. I experienced a problem with some costumes for my kids when I painted one side of the fb and let it dry then painted the other side. acrylic likes to shrink quite a bit, and a heavier coat on one side would really provide a bit more pull to make it warp.

My suggestion would be minwax as you did with wiping off excess. Then multiple light coats covering top and bottom evenly, letting each coat dry between. Easy enough to try on just a bigger scrap piece of dtfb instead of testing on a whole airframe!

 

[danskis]

Ironing the edges adds a lot of rigidity - but doesn't stop warping. Heck, all my dtfb comes pre-warped out of the box

 

[Foamforce]

This weekend I built a Snowball and applied a heavy layer of Minwax with a brush and didn’t wipe off the excess. After 24 hours I put on two coats of paint. That seemed to do the trick for waterproofing. No warping this time. My conclusion is that I just put it on too thinly the first time.

That said, my snowball is overweight. I should have weighed it before and after Minwax and paint to see how much that added. Next time!