"Great Stuff" > not for plastic fins

Finally got around to trying something out I had read about some time ago. Using spray foam in plastic fins to give them strength. I was planning to use the technique on my Estes Honest John and Saturn V.

I cut a jig out of Hardboard (MDF) to keep the fins straight and taped and clamped my Honest John fins on it. I filled the fins with a light coat of spray foam – Great Stuff- and left it over night.
When I went to cut off the excess all the foam came loose. The foam hadn’t stuck to the plastic. I was a little shocked.

Well I guess I’ll be trying something else.

You really don't want to use that kind of foam on rockets anyways. The only foam that should be used is the 2-part foam.

2-part foam will stick to pretty much anything. just sand the plastic lightly to make the surface rough first.