Wascana.
I used the Sikkens log and siding semi stain, not the Cetol product. I have had no issues at all so far. First, I did a single coat to the interior sides of my plywood, then did 3 top coats of Benjamin Moore Marine varninsh. Once the plywood was installed, I sanded the roof and side panels to fit so the edges came out correct. Then a coat of Sikkens and 3 coats of marine varnish. Finally I had the pin stripes put on (painted-not decal). The painter that did the job suggested after a year or 2 we should pull the metal doors off and apply another coat or 2 of varnish. I plan to do that this summer. The reason is that it really is treated sort of like a boat, so this is just a "maintainence" coat.
Bear in mind that you can change the tone by heavy you apply it. I brushed it on and then used a rag to get it consistant. Play with it on a sample board first. AND I am sure that there are better ways to do this, but I am very happly with mine.
Good luck.